Mystery music mark

Hello, everyone!

I have been asked by my choir mistress to rewrite some sheet music from a preview she found on the Internet and I've encountered a mysterious music mark which I have no idea what it means. I tried to google it and look it up here on forum but since I don't know what's it called, I didn't find anything. So I'd like to ask if anyone knows, what this mark means and how to put it into the sheet music using MuseScore.