If someone already figured out this hack, my bad.  I couldn't find anything about how to do it.

I created this homebrew feat: Background Ability Bonus +2/+1  

If you add this as a second granted feat for your custom background, it should let you pick the bonuses you want.

I will also create one for +1/+1/+1:  Background Ability Bonus +1/+1/+1

Here is an image of how to insert as feat in Background:  https://freeimage.host/i/37JPz6F

Here is an image of how to set up the choice of +2/+1 or +1/+1/+1:  https://freeimage.host/i/37JP23u

now to make it work with restricting abilities to choose from. Like only CHA/STR/DEX or INT/CON/WIS

Quote from Enrif >>

now to make it work with restricting abilities to choose from. Like only CHA/STR/DEX or INT/CON/WIS

You'd need a feat for every combination. 

-

Anyway, worth noting this will prevent the homebrew from being shared publicly, if that was the intention. You don't need to share publicly to use it in your campaigns - everyone in a campaign has access to homebrew created by anyone else in the campaign.

Also note: if your Species and your Background do not specify an ASI, you will be given an option to choose your ASI in the Abilities tab. This means this method can confuse people since they'll choose ASI in the feat and then on Abilities tab be prompted to choose ASI again. 

Quote from Cyb3rM1nd >>

Quote from Enrif >>

now to make it work with restricting abilities to choose from. Like only CHA/STR/DEX or INT/CON/WIS

 

 

Also note: if your Species and your Background do not specify an ASI, you will be given an option to choose your ASI in the Abilities tab. This means this method can confuse people since they'll choose ASI in the feat and then on Abilities tab be prompted to choose ASI again. 

 I did not realize when I create the feats that they have some sort of macro that if you have homebrew background it adds the options.  My bad.