what goes here ? I am trying to get an update.

That looks like the auth prompt for downloads. I'm guessing based on the querystring params, but were you trying to download PE for Ubuntu? The credentials used here depend on if you are downloading with a paid license ID or with a Forge ID. If you are using a PE license, you'll use a username

license-id

, but if you are using a Forge API key, you'll use the username

forge-key

. Then the password is the matching license/API key. (EDIT: Oops! Looks like you already got an answer on this!)