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 ":---:".
en dash: https://www.compart.com/en/unicode/U+2013
em dash: https://www.compart.com/en/unicode/U+2014
Edit: Ah, Libreoffice does have a built-in autocorrect for em dash, but you have to type this: