Use your client to find the note id (usually in a menu somewhere) and paste it into the message you are writing.

Like this:

nostr:nevent1qqsqq0mr9gaw6cpn3qrk5fyfvkpczxr2qcz5aqkh8lrnkn47dg62vnqppemhxue69uhkummn9ekx7mp0qgsy3dgwghq6qjdttwsw4dpcru9rk6ep6zd24gdw56c79khj067a02crqsqqqqqp4rjwnc

It usually looks like this when you are typing:

nostr : nevent1 ........ xyz

But without the spaces.