I am a free user of DDB. I just went to look at Magic Initiate to see exactly how they allow a choice of ability scores to cast spells. I started with a copy of MI to supposedly make changes to that feat. There are no options, actions. Nothing for reference. Hitting the + button reveals nothing. Is there someplace that has tutorials for creating feats? I'm pretty sure I have everything correct, at least from reading the ? next to the title of the pull-down menus.

The Magic Initiate feat uses backend coding for function that the homebrew tool we can use doesn't have access to. It cannot be properly replicated using Homebrew tools.

You would need to create a feat that uses Intelligence, one that uses Wisdom and one that uses Charisma. 

For guides on homebrewing see the Homebrew forum pinned threads. A particularly good one is the FAQ : https://www.dndbeyond.com/forums/dungeons-dragons-discussion/homebrew-house-rules/131411-a-homebrewers-how-to-faq