Keep your form as-is (where people can freeform type their own email address), but you’ll need to add a new linked record field in your CRM table that links to the form table.
Then, you would just need to create an automation that takes the newly-submitted form record, searches for that particular email address in the CRM table, and then adds the newly-submitted form record to the linked record field in the CRM table.
(You can add onto the contents of an existing linked record field by inserting the existing linked record field first, then typing a comma, then inserting the new field’s value.)
Scott originally suggested updating the linked record field of the record that was found.
Instead, update the linked record field of the original newly-created record with the record ID of the found record. Because this new record starts with no linked records you only need to update it with the found record and don’t have to mess with adding on to an existing list. Airtable will take care of creating the back link in the other table for you.
Thanks @kuovonne & @ScottWorld for your help, much appreciated!
I think I understood what you both suggested and have made a quick Loom video explaining the solution in case anyone else runs into this issue