They call it tucking. I never tried it and it looks hard to do effectively, but here is the "general method". You need medical tape:
-Shave/epilate the whole area.
-Push up the testes (there's a "path" towards the inside that they can follow until they are hidden without hurting) and tape them there.
-Pull the penis down over the pushed-in testes, towards the butt, and tape it there.
This, if done correctly, should theorically allow you to be seen even in underwear without question unless very close inspection is performed. However, it does require daily shaving or epilating. Shaving there every day is very hard (well, I personally could never do it more often than once a week, partly because of the time it takes, but also because my skin would tear up from so much irritation), and I don't even know if it's possible to epilate this area in "men".