Hi Everyone,
I worked out what to do to invoke this.
I ended up using "system.util.sendMessage()" instead of "system.perspective.sendMessage()", parsing the "sessionId" and "pageId" in a payload, and using those parameters in the "system.perspective.openPopup()" within the Session Message Handler.
Just for reference, see screenshots of working code:
Script for "onActionPerformed" event on Button:
Script for Session Message Handler (breakStartPopup):
I hope this can help anyone in the future.
Note:
I used the following link which really helped getting to this solution - Perspective Session Event Scripts | Ignition User Manual