passing Form parameter "ByRef"

I have a form that is going to be used as a subform on another form. I want to create a method on the subform that will be called from the "parent" form that will pass "itself" as a parameter. Then, the child would store that reference and be able to use it to call methods in the "parent" form. I put the pieces in place, but when I attempt to save the "child" form, I get a message relative to assigning the parent reference (a VAR parameter passed to a global function in the "child" form) that states "Assignment is not allowed for this variable".

Are there any ideas how I can accomplish the above?