Tracking slide completion when using 'NEXTCHAs'

Hello, All.

David Anderson put together a superbly helpful article on using NEXTCHAs here: https://community.articulate.com/articles/nextcha-prove-learning. I've put some of these into practice using the PostRoll template. 

I need to track learners' completion of each slide (to allow them to access the quiz), and am trying to figure out how to set the variable for the slide to 'true' when advancing via the NEXTCHA button instead of the Next button.

I've tried this:
'Adjust slide variable to true when user clicks NEXTCHA button, on condition that PostRoll is equal to (desired input)'

It doesn't work, as I can't get that to appear in the trigger list *after* the PostRoll trigger.

I need these slides' variables to be true, as that triggers a checkmark on a summary showing which slides they've completed. 

Any suggestions? Thanks!

Allan