Depends on the client as to how you do it. Some you can just paste the note ID into your note, and others you need to put an @-symbol before it. Like this below. I’ll screenshot it too so you see what I typed:

nostr:note1dfdr3u38p03ux8zesjhemhcwd0nks225kq4quxw5nenfdqd4c35qlp5tx9