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