I added a Feature for sub-class spells and the spell shows up, but instead of the red Cast button it says At Will. What setting do I have to change to get the Cast button?

If it’s actually part of a subclass feature, then you need to set “consumes spell slot” to “yes.”

I'm trying to do this with a magic item. I want to add a new homebrew spell to a PC's spell list which they can cast with a Warlock spellslot (no charges on the item). Any ideas?

Can see Sposta has answered this in many threads already, thank you for your help.

Hope the devs add this as a feature soon. Saw people asking for this back to 2017. 

I added a Feature and added the spell Spells sections as you can see in the picture bellow and already had it set that way and yet it still displays as At Will.

https://1drv.ms/u/s!AmoO9upBR5nRqyZMvgCIsTmRIGcs?e=EgCr4f

I guess it is a matter of having to go into the Character Builder, selecting another sub-class, and then going back and re-selecting to force it to update.

Quote from JeffBomba >>

I guess it is a matter of having to go into the Character Builder, selecting another sub-class, and then going back and re-selecting to force it to update.

 You could also change the character level and then change it back. That also forces the update.

Quote from JeffBomba >>

I added a Feature and added the spell Spells sections as you can see in the picture bellow and already had it set that way and yet it still displays as At Will.

https://1drv.ms/u/s!AmoO9upBR5nRqyZMvgCIsTmRIGcs?e=EgCr4f

 You don’t need to have “Sorcerer” in the class section as that would only be necessary it they got to “choose a spell from the Sorcerer spell list.” Also, you don’t need both “counts as known” and “always prepared” filled in, every class only uses either one or the other, not both. If they have a limit on how many spells they can learn (Bards, Rangers, Sorcerers, and Warlocks) then they need “Counts as Known” designated as either “yes” or “no” depending on if it’s supposed to take up one of those spots or not. If they have a limit on how many they can prepare each day (everyone else) then they need “Always Prepared” designated.

Quote from WinR >>

I'm trying to do this with a magic item. I want to add a new homebrew spell to a PC's spell list which they can cast with a Warlock spellslot (no charges on the item). Any ideas?

 Does it have to be an item?

You could do it by making a customized version of either their subclass or race/variant/subrace. But those are the only place where you can access the necessary dropdowns. (“Consumes Spell Slot->Yes,” “Counts as Known->No”)

Thanks Sposta. I'd prefer it to be part of the item. I think I'm happy with where I've got to for the time being. The spell shows up in the spell list, but it is annoying it says "at will" instead of cast. I will speak to the player to say there are limits to what DND Beyond can do and hopefully that is all that will need to happen. 

I do wish they'd add more flexibility to the modifiers though to allow this to work. Feels very thematic for an item to give you special knowledge. I really worry about giving my players to many items that they can cast spells from as it will be impossible to drain their resources.

I'm trying to do the opposite. I'm trying to make a feat where it gives you every spell to cast at will. Then I can just play around with having every spell to cast at will. I just can't get the spells to appear at will. Either they just don't show up or they have charges.

You have to set the Spellcasting ability, and you have to leave the number of uses fields blank. That should work, but they’ve been doing some work on the backend for feats so it might not anymore.

What do you mean by setting the spellcasting ability? How do you do that?

Also, whatever you do, make sure that you DO NOT check the “Is Infinite” box, it doesn’t do what you think it does.