Make sure you have ~/.ssh directory and has correct permissions:
mkdir -m 700 ~/.ssh
Create your keys:
ssh-keygen -t ed25519 -a 100
if you press enter to accept the defaults, your public and private key will be located at ~/.ssh/id_ed25519.pub and ~/.ssh/id_ed25519 respectively
cat ~/.ssh/id_ed25519.pub
copy the output of the last command and paste it in the sshkey field on the signup form