Hey all, thank you for reading this. Just a heads up I dont have any books on DND beyond so I cant just pull up another subclass to see how they do this.

My question is this, I want to give a homebrew subclass the ability to add either its strength or dex mod to its spell save DC and spell attack rolls.

The only problem is that I cant figure out how to do that. Here is what I have so far:

Modifier: Bonus

Modifier subtype: either spell save DC or spell attack, one for each modifier option

Ability score: Either strength or dex depending on which option the player chooses

And then I left all of the defaults as they were. though I did remember to click on the is granted box and set it so that the player gets a choice between the two when they reach level 9 (when the ability is granted). The problem Im having with this is that I can get it to open the drop down menu but its not sticking. So like I choose dexterity and it will select but when I change tabs in the character creator it goes back to being unselected.

You cannot. You would have to list it as plain text and the player would have to adjust things manually on their character sheet.