Get mouse position on click in comp preview

I want to get the position of the mouse when the user clicks in the comp window. I can get the mouse position using OS calls, but I want the relative position, exactly like PF_Param_POINT does it when you press that target button next to the parameter value.

I though about geting the size and position of the comp and then calculate the mouse position using the OS mouse calls, but I don't know how to get the position and size of the comp.

Another option would be to add a hidden PF_Param_POINT, and force the activation of the pick tool from the parameter, but I didn't find a way to activate it without user input.

Any ideas?