When a user creates a record in the portal, it will be automatically linked to them. They don't need to select themselves from a link records field (nor do you need to show the field on their form).
The linked record of the user does not get pre-filled in the form when you are adding a new record, it gets added to the record after it gets created. You will be able to see in Airtable that the user is linked to the new record.
We highly recommend not adding the "user linked field" to the form. Please remove it if you have it added already and it's causing confusion.