For the Character Builder, it usually only goes by what is available for normal rules, unless you get really complex in homebrews. It can at times be modified on the final character sheet, but not always. My character has gotten some campaign rewards, a free feat at lvl 1 (which includes a permanently prepared spell), and an additional cantrip. Is there any way to add this to the character sheet? So far I haven't been able to prepare the spell or add the cantrip.

I think the only way to do this is to homebrew the Feat.  There's currently no overrides to the spell system like there are with Ability Scores.

If you were granted a feat then create a feat and add it.

To add extra spells without a feat, go to manage spells, click on the spell you want, and click add at the bottom (I dont think this works on cantrips, may still need a custom feat for that).

Make a copy of the Magic initiate feat and edit as necessary for the spells you wish to add to your character. 

We need a way to add a spell to someone.

I agree. Am having the issue of taking over an NPC that was cooked up for a campaign and then given to me to play. I'm moving everything over to DNDBeyond, and digging it, but this is a bummer. Arguments that it keeps people from cheating is silly if you still have the ability to add all kinds of items, change ability scores, give proficiencies in skills, etc at will. 
Maybe have an "override" section, so people know if they're going in there, they're fudging the rules?

Can always just edit the subclass and add whatever spells or spell lists you want and choose which are "known", "prepared", "at-will", "limited use" or "uses spell slot". You can even overrule some spell aspects like casting time.

Seems simple enough.

I'm trying to homebrew it. It says I am required a modifer subtype, but it does not give me any options to choose from. I can not type one in, and it is preventing me from finishing the homebrew. Suggestions?

Quote from Phantom_Ashes >>

I'm trying to homebrew it. It says I am required a modifer subtype, but it does not give me any options to choose from. I can not type one in, and it is preventing me from finishing the homebrew. Suggestions?

 To add a spell it isn't a modifier, you just click add spell.

No, I am trying to homebrew a feat for extra spell slots.

Quote from Phantom_Ashes >>

No, I am trying to homebrew a feat for extra spell slots.

There's no way to add spell slots.

Quote from Phantom_Ashes >>

No, I am trying to homebrew a feat for extra spell slots.

 Then why did you post in this thread about adding extra spells known?

Quote from DxJxC >>

Quote from Phantom_Ashes >>

No, I am trying to homebrew a feat for extra spell slots.

 Then why did you post in this thread about adding extra spells known?

 There's a big difference between "adding spells known" and "adding spell slots"

For anyone else looking to solve the original problem of this thread, as I was, here's my workaround: create a homebrew magical item that grants the spell as at-will.

The player will still have to use their spell slots as normal.

How the item will look for that player (make sure they activate it with the red box)

How the spell will look in their spell list:

Quote from brningpyre >>

For anyone else looking to solve the original problem of this thread, as I was, here's my workaround: create a homebrew magical item that grants the spell as at-will.

The player will still have to use their spell slots as normal.

How the item will look for that player (make sure they activate it with the red box)

How the spell will look in their spell list:

 Or a homebrew feat, like the first 3 comments suggested. Works the same way except isn't in inventory, can't be toggled off, and is more out of the way.

This may or may not have been resolved, but I just ran into this myself and it seems DnDB foresaw this. Manually adding the feat and then clicking Manage Feats brings up the list.  You can then drop it down and choose the spell you want (we found this via Spell Sniper for free at lvl 1).

How do you make a feat that adds an additional specific spell known to a Sorcerer, that works exactly like any other known sorcerer spell?

I made a homebrew feat that simply had the specific spell added, with no modifiers to the spell, but that added the spell only as an "at-will"-spell and not upcastable (the spell is upcastable).

To be concrete I want to give Jim's Magic Missile to a Wild Magic Sorcerer, as simply an additional known spell.

Quote from Asherett >>

How do you make a feat that adds an additional specific spell known to a Sorcerer, that works exactly like any other known sorcerer spell?

 

 
You can't.

To add a spell that works with normal spellcasting you need to edit the subclass and have a feature (you can hide it if needed) that adds the spell there.

Do you need to create the character over from scratch to do this? Is there a way to just "edit" it and not have to homebrew the whole subclass again? I'm trying to give an extra spell to one of my players, I don't really want to fully recreate their character. 

You can just edit. I made this video here to show you how.