I'm so glad you figured it out and got it working!
I made it sound like using the instanceID from the original registration form wasn't a great idea but really, there's no reason not to. It ends up being very closely related to the third alternative option that I listed. The most intuitive key for you is probably the best one to use! 
If you have any specific questions, don't hesitate to ask. You may be interested in some new documentation that we recently posted. We've also been doing office hour sessions with Insiders.