There is not a specific button, there are several ways to nutmeg a player but it always involves (some) luck. I found that performing the shot fake (square and then cross) will often result in a nutmeg. The defender sticks out his leg and the ball will go through.
Wel u gotta do some dribbel moves,freestyle with the R2 and L1 button and use the d-pad or analog and just wait for the right moment,but i have to say doing a nutmeg in we10 is way harder then in we9 ,defenders AI is a lot better and yes u gotta have some luck with it.Try to do it against a low class defender from erendivisie or something.
How ever i did managed to do it with Ronnie on gattuso
But it took me very long time to do it.