The part I was having trouble with was how to set the value of the family field to what I want. What I ended up with is using a formula field in the family table to build the url I want. ‘/add-address?family={family record ID}’
Then I set the value of the hidden field based on a url parameter and use the “family” parameter. This seems to work although it’s a bit buggy in Softer databases. I’ve submitted a different issue for that: Softr Databases: Invalid data can be added when submitting a form with related record
Thanks for your help.