if all your solution requires is to add brackets to a fermata, there’s no actual “designing” to do… it’s just “insert left bracket”, then “insert fermata”, and finally “insert right bracket”.
I just tried, and it’s not working for me either.
Try this instead. In my file select the playing techniques.
Select the fermata and click the pencil to edit it.
Click the star in the lower left corner. Next time you open
your own file it should now be available among the playing techniques