What is the correct and alternative way to (post) action on "Purchase Order Subform"?

hi…
I want help, I want to put a procedure to check an input value (whether it was entered or not) on Action (Post), and on “Purchase Order Subform” but it gives me an error because (Post) Action is not found(or included) in “Purchase Order Subform”.

What is the correct and alternative way to (post) action on “Purchase Order Subform”?