You could just ensure the other party has a key and then send them encrypted text data they could decrypt on their end.
It's not exactly trivial to add support for this automatically in DMs because of sharing keys between users and ensuring the right keys are in use, considering it would have to be possible to rotate them at any given time.
If you need encrypted communication, use something designed for it. That's probably the best way to go
I'm on a pain management plan that includes an opioid. It requires mandatory 3 month check-ins with my doctor, and my previous pharmacy had a pharmacist that would ask me if I "really needed it". That pharmacy delayed my refills so much sometimes that I would go into withdrawal. And if I called to ask what was going on, they would start asking me why I was so concerned and if I was actually using them.
New pharmacy only has an issue if I pre-emptively request a refill, but even then I just explain my work schedule and issues with remembering to submit the refill and they just hold it until the normal refill date.