The password (or any login field) would be a column in your users table. So you would make your own logins in Airtable. That information would be used to lookup records and would not be provided by the extension itself.
In Airtable you can add a formula field for password and use it as a lookup field. That way anyone who uses your client portal would have to enter that password correctly or otherwise the client portal won’t work. But normally our client portal customers would allow each user to have a different password. You’d add a new text field for passwords in the users table. That way every user could login using their own password.