If I had a nickle for every time a student asked me how to cry on command, I would be a rich man. Asking about how to cry on command is like asking how to take off and land a plane on your first day of pilot school.
There's a foundation that needs to be laid down with certain techniques before "crying on command" is an option. Here's the thing...when you learn how to do it the right way, you'll realize that how to cry on command is barely the question at all. The question is how do I act truthfully in the given imaginary circumstances? If crying is the right emotion, you need to feel it, not fake it. We talk about this extensively in Meisner training.
Crying on command is possible but it's not what you want to focus on, it's about how you get there. Watch the video below to see what I said when someone asked me how to cry on command.