I made a custom warlock invocation for my players. I have no idea how to put it into DND Beyond. I don’t think you can create them in DND Beyond yet. Feel free to use it and post your own custom invocations.

Word of Creation

Prerequisite: 18th level
You can cast Wish once using a warlock spell slot. You can't do so again until you finish a long rest.

Homebrew invocations, fighting styles, etc are planned.

Your invocation doesn't make sense. You can cast a 9th level spell with a 5th level slot. The invocation should let you cast wish with your 9th level mystic arcanum.

I agree, we should have a thread to discuss hombrew invocations. A few posters started a similar thread:

https://www.dndbeyond.com/forums/dungeons-dragons-discussion/homebrew-house-rules/2307-homebrew-warlock-invocations

There are plenty of good invocations in this old thread. Let’s see what else we can come up with.

Soul Offerings.

Prerequisite; lvl 15.

You've learned how to please your Patron with gifting the Souls of worthy foes and powerfull creatures, giving you in return more power.

When you reduce a creature with a CR equal or higher to your level to 0 HP with an attack or Spell, you can then decide to Offer their Soul to your Patron.

When you do, you either regain One spell slot that you've previously used OR you gain an Inspiration , but this Inspiration must be used before the next 24hours, or else this Inspiration will dissipate and be lost.

A soul offered in this manner is lost and cannot be used for any kind of action or spells that requires the target having a Soul, the only way to restore a Soul that as been offered is through a Wish spell

It require no action, and you can do it once per short rest.

This invocation was posted by user  Thain:

Dreaded Hexes

You can cast Hex at will, without expending a spell slot or material components. In addition you may choose for Hex to deal psychic damage instead of necrotic.

I like the idea but I think it needs a little tweaking as it seems OP for a Warlock to cast Hex at will. Most of the at will invocations include buffing spells like mage armor or false life...not powerful damage spells. Here is my attempt at balancing:

Dreaded Hexes V2 (Cursed Memory)

You can cast Hex once, at its lowest level, without expending a spell slot. You can’t do so again until you finish a long rest.

This version would be more useful at lower levels when Hex only lasts  for an hour. At higher levels it would just be insurance for those time when you lose concentration and don’t want to burn another precious spell slot. 

Thoughts on either version?

In my Fellozial's Ultimate Guide to Poison, I include 55 homebrewed Eldritch Invocations distributed among customization options for 16 named warlock patrons. Here's a sneak peek at a few of them:

Available to Warlocks of Shaktari, Queen of Poison (a Fiend patron):

• Aura of Vudra. Prerequisite: 5th level. Creatures within 30 feet of you automatically lose any immunity to the poisoned condition they may possess, and any such creature's immunity to poison damage becomes mere resistance while within your aura.
• Riposte of the Marilith. You can take an extra reaction each round, but this reaction can be used only to make an opportunity attack.

Available to Warlocks of the Teeth of Dagoaxath (a Hexblade patron with ties to Triton legend):

• Deep Warrior. When engaged in underwater combat, neither your Hex Warrior weapons nor your pact weapon (if you choose the pact of the blade) suffers from disadvantaged attack rolls regardless of the type of weapon. If submerged in water while wielding your Hex Warrior and/or pact weapon, that weapon also deals 1 extra die of damage on a hit.

The Deep Warrior one might be a fun one to try out in a Saltmarsh campaign... ;) 

Quote from Chaosrex >>

Soul Offerings.

Prerequisite; lvl 15

When you reduce a creature with a CR equal or higher to your level to 0 HP with an attack or Spell, you can then decide to Offer their Soul to your Patron.

When you do, you either regain One spell slot that you've previously used OR you gain an Inspiration , but this Inspiration must be used before the next 24hours, or else this Inspiration will dissipate and be lost.

I really like the idea of offering souls to your patron by how often do you actually kill a CR 15+  creature? Also, isn’t inspiration a DM mechanic ?

Based the CR on the approriate level of the Warlock( prereq. lvl15), but that might be a tad ambitious, maybe reduce it to CR 10.

And the inspiration bit was something of an alternate way to use the invocation, but might as well go with a simple reroll for attack or saves.

Relating back to the OP, I add Custom Invocation to D&D Beyond as Feats. When the Warlock Class requires me to choose an Invocation and it is not one of the ones in the D&D Beyond list, I just keep that selection unselected and add the desired feat instead. I prefix the name of such feats with Invocation so that it is obvious that it is a Invocation. Feats have access to all of the D&D Beyond features like Modifiers and Actions so you can accomplish most things via feats.

Hi, I was thinking about an invocation idea and would like to know your opinion about it.

Loyalty Demonstration

Prerequisite: 10° level and over 100 hp
It cannot be a favored soul.

1 time per turn, using your action, you can sacrifice 20% of your total HP taking as neurotic damage (cannot regenerate HP until end of next turn) and Regain 1 Warlock slot.

obs: (I don't know how to speak English very well so if you have any errors in writing that change something please let me know.)

is there any way to add homebrew warlock invocations in d&d beyond so that I can select it in the app?

No. There is no way to create homebrewed Invocations on DDB. That’s a base class feature, which we cannot access.

Quote from JoshuaRobertson7101 >>

is there any way to add homebrew warlock invocations in d&d beyond so that I can select it in the app?

 I've heard (a long time ago) that stuff like that is "planned", like they will have it before they stop updating DDB "planned."

But a lot has changed since then.

I just make Custom Feats with the respective invocation specs and add the feat when my warlocks gains new invocations.

Imbue Life Essence
Requirements: Pact of the Chain, Level 5
When summoning a familiar, the warlock can imbue the familiar with a small shard of the warlock's essence. If imbued this way, the warlock can cast any spells with a range of "self" on the familiar as if the range was "touch" instead. If the spell requires concentration, the warlock performs the concentration. When a familiar is imbued this way, any damage that the familiar takes is also dealt to the warlock (after all damage immunities and damage reductions are applied) but any excess damage is ignored. 

Greater Imbue Life Essence
Requirements: Imbue Life Essence, Level 10
When summoning a familiar, the warlock can imbue the familiar with a small shard of the warlock's essence. If imbued this way, the warlock has the option of applying any "self" spell, cast on the warlock, to also include the familiar as long as the warlock can sense the familiar. The spell is still a single spell, with concentration supplied by the warlock if required, but affects multiple targets. If either the warlock or the familiar meet any conditions for ending the spell, the spell ends on both targets.