Custom Submit Button for Quiz Question... on the Master?

Hello!

I've followed lots of posts about creating a custom submit button, and I'm there - it works great. But it seems like it has to be pasted onto each slide, every time I create a new question. 

I have a Question Master that works great - the custom Submit button is on it, and it has the appropriate trigger. 

However, because the "Submit Interaction" trigger seems to require the name of the interaction and the name is slide-specific, the button on the Master has "Submit 'unassigned' Interaction" and it doesn't work... and because it's on the Master, I can't edit it on a slide by slide basis even to select the correct interaction.  (Though I'd rather not have to do that either...) 

Any help would be greatly appreciated!

Thank you!

-Sarah