Easier than I thought.
Somehow, I messed up when I tried the same thing before asking the forum.
Now its working.
Many thanks for your help.
How you do this for a button is slightly different from how you do this for a button bar. I had to chase this one around the tree a few times a month back when I was experimenting with setting up a vertical button bar to take the place of a drop down list in order to get a better UI for use in FileMaker Go.
There are two types of buttons, now. The old button and the new button bar.
New Button Bar $$Variable
OLD button <<$$Variable>>
Took me a while to get used to this...