It shouldn't be asking for a password if you're trying to access the test server. Unless you're trying to access the free weekend client, which has been closed.
1. Hop on your PC (hopefully it's a gaming PC otherwise you might have a rough time)
2. Login to your console-linked uplay account on the uplay client that you hopefully have downloaded
3. Check the "my games" tab and search for something that should say "Rainbow test server"
4. Install
5. Play