Depends on the client you are using. You can either copy and paste their npub, or if your client might allow @ing someone. Amethyst does this, for example.

On most clients you can start typing @ and if that user is cached in the search they might popup. Otherwise you can also do nostr:npub... And it'll render the tag in most clients @OceanSlim

Thank you @OceanSlim and @Jordan Eskovitz Fast responses to help answer such a basic question from myself.⚡ ⚡

Do @npub1dst0...vf8l that is bit @ in front of the thing.