Forms only create new records; they don’t update existing records. So you would need to use an automation to do this.
The exact solution depends on the primary field in your users table. For example, if your primary field in the users table is an email address, then make sure the user enters their email address into a normal email field on your receipts form.
After the receipts form is submitted (I.e. the new record is created), use an automation to copy the email address into the linked record field that links the receipts table to the users table.