The registration form for the pubnix can only accept one ssh key. This means that, incase you need another key, or you have a key in a format that isnt supported by the form (anything that isn't rsa/ed25519/ecdsa), you need to input one key in a supported format first, and then modify it later after first login.
If you don't have a key with a format that is supported, go through the Generating an SSH key to create a supported one.
After you ssh with the key, run the following
$ nano ~/.ssh/authorized_keys <the ssh key you gave for signup> <add your new key here> Control+x
Note: If you have multiple keys, you might need to use the -i flag to specify the right key to use like this:
$ ssh -i ~/.ssh/id_keyyougaveforpubnix USERNAME@p.projectsegfau.lt