There actually is a “matrix user” field when you edit your profile; if you put something there you’ll have a “Send secure message” button on your profile that does something with matrix.to. See https://lemmy.ml/u/dessalines for an example of a user who has this button.

Also note that the unencrypted direct messaging feature here has a ⚠ which says “Warning: Private messages in Lemmy are not secure. Please create an account on Element.io for secure messaging.”