You mean quoting a note? Just copy the note id and paste it in your note. Make sure it starts with `nostr:`.

nostr:nevent1qqs0ql8zt4tf4tpnayljfgnqg38r4a8aesuckfe7ydws3qmq5z4xfecpz4mhxue69uhhyetvv9ujuerpd46hxtnfduhsygx575qsx6xfnuftt5sx9ggq9kaa8s4uy4ttdsjpfyd4rca5vzkeu5psgqqqqqqspyd6xs