Using RT's CLI to create multi-line comment from script

RT’s CLI lets me add a one-line comment to a ticket:

rt comment -m ‘ignore this test’ 12345

or attach a file as a comment along w/ a one-line comment:

rt comment -m ‘Test attachment’ -a /tmp/test.txt 12345

but how can I add a multi-line non-attachment comment?

I’m running this from inside another script, so bringing up an editor
isn’t feasible.