You can set up a login field to show a list of searchable existing values from Airtable rather than just typing it. This way, the user can just pick a value to use during login. You can set this up by following these instructions:
Go to the Share (for Forms), or Users Table & Login (for Portals)
Go to Login Fields
Click on a login field where you want to use this feature
Turn on Allow users to search from existing values in the table toggle