> If one prefers not to use OneDrive (or other online storage)
I would probably try to do this by copying a plain text file (.txt) over
a USB connection. Plug the phone into the computer and a notification
should appear on the phone giving options for what to do with the USB
connection. There should be an option like "transfer files" or perhaps
"MTP" and when you select this the phone should show up as a storage
device on your computer.
Then you can move or copy a text file you've created onto the phone,
find and open it using the phone's file browser app, and then copy
contents to paste into your messenger app.
Jakob