Now that I mostly work on dedicated platforms like Pantheon, Acquia, and WPEngine, it’s been a while since I’ve had to manually add an SSH key on a server. Most platforms expose a web form on your profile that takes care of that for you. If (like me) you’ve forgotten how to do this, Digital Ocean has a pretty straightforward guide.
One thing to watch out for: permissions matter! I added a key on the remote server, tried logging in, and was still greeted with a password prompt. Turns out that I needed to check the permissions on a number of directories, as outlined in this Q&A post:
Your home directory ~, your ~/.ssh directory and the ~/.ssh/authorized_keys file on the remote machine must be writable only by you: rwx—— and rwxr-xr-x are fine, but rwxrwx— is no good¹, even if you are the only user in your group (if you prefer numeric modes: 700 or 755, not 775).
Once I adjusted the permissions, I was able to SSH in cleanly with no password prompt.