That's an en dash, not an em dash. An em dash is longer and as far as I know Libreoffice doesn't have a built-in way to make one (though you may have added it to the autocorrect settings yourself).
Thanks! I did confuse the two despite knowing of both.
So ":---:" does work for the em dash? I thought something with fewer keystrokes work, too, at least I remember the em dash from less, but perhaps I just typed it so quickly I did not realize it was indeed ":---:".
I don't think the character is that uncommon in the output of slightly-sophisticated writers and is not hard to generate (e.g., on macOS pressing option-shift-minus generates an em-dash).
In fact, on macOS and iOS simply typing two dashes (--) gets autocorrected to an em dash. I used it heavily, which was a bit sloppy since it doesn't also insert the customary hair spaces around the em dash.
Incidentally, I turned this autocorrection off when people started associating em dashes with AI writing. I now leave them manual double dashes--even less correct than before, but at least people are more likely to read my writing.
