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

nostr:nprofile1qqspdudqzqx5ellme3prp68qus5se3vynsddcexkv5la5p7qxxcswjcprdmhxue69uhhw6r9v96zu6rpwpc8jarpwejhym3wvdhj7qgnwaehxw309ac82unsd3jhqct89ejhxtcpzdmhxue69uhk7enxvd5xz6tw9ec82c30hq9nk4

Do nostr:nprofile1qqsxc9h34kzr3jy3usa009yk283m67lnuwdpf7xpc76clrdlv4vy4jspr9mhxue69uhhyetvv9ujuumwdae8gtnnda3kjctv9uv8tvsv that is bit @ in front of the thing.