How do I make a procedure that opens a GUI when player right clicks a mob?

I was trying to make some kind of walking chest mob to my mob, but then I faced a problem: I don't know how to make it open a GUI every time a player right-clicks it.
It seems like there is no option to open a certain GUI using a procedure, then how do I make it open a GUI?

I am pretty new to MCreator and sorry if I don't understand some sort of a thing. Also sorry for my horrible grammar, English isn't my native language.