What you describe does not add a record, it modifies data in a field.
Why is the Procedures field many lines of text in height?
Are you adding the selected text to the text already in that field? That can be done, but there are likely much better design options for what you are trying to do here.
This script step can be the action for your button, but please keep in mind that this is implementing somthing that is likely much less than an optimum design for your database:
Set Field [Visits::Procedures ; List (Visits::Procedures ; Visits::ProceduresSelect ) ]
This appends the contents of ProceduresSelect to the end of the list of procedures already present in the field.
But I think you need a related table of selected procedures and a portal to that related table instead of simply putting text in a large text field as is currently shown on your layout. I suggest looking up "portal" in FileMaker help and any training materials you might have as the next thing to research for your database project.
Genius! Thank you PhilModJunk - it works!!
Task almost complete! ;-)