I am very new to Filemaker. I have a master name file which includes a key field "SSN" I also have several other databases that I want to link by "SSN". I want to display the master name file layout, press a button to select another layout in another database that creates a new record and places the "SSN" in the new record as the key field.

Use a portal with the "allow create related records" option selected in the relationship. Easiest method because FMP performs the work.

Otherwise you need to script the process: "remember" the master record's key field, go to the related database, create a new record, paste the remembered value into the local key field.

The "remember" can be achieved with copy/paste, global fields, or lots of other ways.