I've been testing what I learned from the other thread this morning and it's a solution for me. By the way, our portal is still 10.8 or 10.8.1, so still using the original map viewer. The trick is how you get to it:
After setting up forms for a map, open the map only using the “Open in Map Viewer” button on the Overview tab of the FM web app. Can add layers, fix popups, etc. there. Close it with the big X in upper right corner and you're back to the form set up.
As a bonus, I just discovered default values: in the FM web app, Content > editable layers > choose one and click the Templates tab in the middle. Click on the form below that and you can set defaults in the righthand panel.