What goes here is a 10mm core (or welch) plug. I have replaced a few on my bikes. The available plugs are a bit tall so time with the file was needed. Some RTV paste (sometimes) helps, but only the smallest amount!
One of my bikes despite changing this plug still leaks, because some previous owner mashed up the internal wall of the recess trying to put it in backwards! This oil leak also annoys me!
But yes- 10mm core plug should suite you right (i’m guessing it’s missing?), but also make sure not to punch it in too far as it will touch and interfere with the shaft inside (for gear change mechanism)- because, at least for a little while, the gear change might be a bit stiff if you go too far! 🙂