When using publickey auth method, you don't have to provide anything (except for, obviously, username and hostname), you have to temporarily trust thimbl.net's key. I.e:, the process is:
1. thimbl.net provides its public key.
2. You add it to ~/.ssh/authorized_keys (or whatever your SSH server uses).
3. ...
4. That's it (remove key if you don't trust thimbl anymore or thimbl may even remove it by itself, at the end of setup process).