Tested on OpenBSD 6.3 and macOS 10.13

Host repositories on GitHub

Generate a new SSH key pair and copy public key to the clipboard.

$ xclip -selection clipboad ~/.ssh/id_ed25519.pub
$

Open your GitHub profile.

On Personal Settings > SSH and GPG keys page click New SSH key.

Type-in a Title, for example, a hostname of your computer.

Paste your public key into Key textarea.

Click Add SSH key.

Expect email A new public key was added to your account from GitHub.

$ ssh -T git@github.com
Hi romanzolotarev! You've successfully authenticated, but GitHub
does not provide shell access.
$

GitHub makes your public key available via HTTPS.

$ curl https://github.com/romanzolotarev.keys
ssh-ed25519 AAAAC3NzaC...
$