This option can be used to skip the login page. Prefilling a login field works the same way as prefilling a field in a form. The only difference is that instead of the word "prefill", you should use the word "login". This works for both form and portal. By using this feature, the login page will only show up when there is an error during login.
You can create a formula field in Airtable to generate this link. You can get the login formula under Form Table for the form and Users Table for the portal.
If your goal is to skip the login page and take a user directly to a specific record, it might make sense to use the editing formula instead. It generates a direct editing URL for each record.
On the legacy site app.miniextensions.com
There’s an option to add a formula that generates a unique URL for every record in your table. Allowing users to skip the login page and go directly to their personal view. You can find it at the bottom of the extension settings page.