That was years ago for UE4.2 or something like that, i have backups somewhere, with a bit of luck i can dig it out and make screenshot.
It is really just some simple trigonometry. Draw triangles from camera location, to mouse cursor and where real location on floor plane should be.
Built in calculations for real mouse location was picking door frames, some decorations etc, and i wanted REAL location on floor plane for AI, so i made that trig calculations myself.
Will try to find it.
Small update: i think i have that project, but it was done with dungeon architect plugin and its 4.11 version of engine. (cannot be loaded with other versions unless i have dungeon architect for it). So i will try to get it for 4.26 or install 4.11.