CTRL+M is the mirror command, then you must pick the point you want to mirror about.
In this case choose a point in the middle of the window.
With windows and doors you just need to pick one point to mirror as it will automatically mirror within the wall.
With any other element you must choose 2 points (i.e. a line about which the mirror occurs).
One of the forum moderators.
Versions 6.5 to 25
Dell XPS- i7-6700 @ 3.4Ghz, 16GB ram, GeForce GTX 960 (2GB), Windows 10
Dell Precision 3510 - i7 6820HQ @ 2.70GHz, 16GB RAM, AMD FirePro W5130M, Windows 10