Depends on your client, really.

On Primal, you can either type @ and a dropdown list will appear that searches for a user, that you can then click. Alternatively, the most working way is to use nostr-links; basically:

nostr : pubkey

(remove the spaces)

When you start something with nostr: and add something like npub... or nevent..., then you create a Nostr link, which most clients will render as clickable!