How to click a button lying on a form thru a short cut menu. On doing so, I face an error "THIS can only be used within a method."

The actual command is 'This.parent.cmdSales.Click'

A menu isn't a child of anything, and what's more, isn't an object, so THIS.Parent is never going to work in a menu procedure.

A menu hit is more like an ON KEY interrupt, you can only address things that are in scope. So you must obtain a reference to your form through some other mechanism. Many people use _screen.ActiveForm or similar, i.e. the command in your menu procedure would be

_screen.ActiveForm.cmdSales.Click()

I'm not saying this would work for you; I'm saying you need to find another way to address your form.

I hope this helps,
- Colin