How to pull information from external site into piped variable

I have a survey that pulls up an external site (of my own) to do part of that survey in an iFrame in qualtrics. That code works brilliantly. But I'm struggling with figuring out how to tell qualtrics that the iFrame part of the survey is done so that I can turn on the "continue" button and allow subjects to keep going.

The external site is written in a combination of JS and PHP and stores all data to a mySQL database. At the moment, I've been using it by having qualtrics automatically turn on the continue button after a specified amount of time, and having my external site end on a "You can now continue" screen. But quite often, subjects see the continue button turn on and skip the last part of the external experiment. Is there any way to get the URL address of the iFrame in qualtrics on a frequent basis (or any other method) so that I can read when they are done with the experimental part and turn on the continue button?

Thanks, and any help and guidance is greatly appreciated!