You can set up a login field to show a list of searchable existing values from Airtable rather than just typing it. In this way, the user can just pick a value to use during login. You can set this up by following these instructions:
Go to Users Table & Login
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
This option is enabled by default for new login forms & portals. For testing, you can use the magnifying glass icon to log in as different users. You can disable this in the Users Table & Login section.