Below is probably the quickest way to achieve this
1. Generate SSH key (if you don’t have one already)
ssh-keygen -t rsa
2. Use SSH to create a remote directory ~/.ssh
ssh email@example.com mkdir -p .ssh
3. Append your public key to .ssh/authorized_keys on remote host
cat ~/.ssh/id_rsa.pub | ssh firstname.lastname@example.org 'cat >> .ssh/authorized_keys'