Update - I've added an action parameter which gets the value I need from the source screen, but when I try using PARAM(1) on the window, it says the variable doesn't exist?

Resolved - added the code to the screen code SPE script instead of the action script.

Thanks for circling back and sharing nickdavies07 

No problem. I've also put together a blog post detailing the steps on how to do this from start to finish, just in case anyone is interested.

medium.com/.../launching-pop-up-windows-and-passing-data-in-sage-x3-bccf3c7679a4