Send a private message - how?

How would I send a private message here?

I’ve seen several mentions in posts that one can send confidential details in a private message.
How would I do that?

Asking the internet reveals that one needs to earn a trust-level badge before the envelope button appears on people’s profiles.

I don’t see that button even though I got the basic trust badge.

Any advise is most welcome!

I mostly use the private message feature to ask for private details like VAT numbers and addresses, if anyone wants them updated on their invoices, and I doubt they’re used for much else. I don’t think they’re enabled for anyone other than moderators / employees right now. Is there anything specific you’d want them for?