How do I set a field in admin product-information page to "required" ?

[Genius WebDesign]

Hi,

I´m looking for a way to make one of my fields in the admin produkt information page "required".
Specifically I want the "Product SKU" field to be required, meaning that when I click "save", and if nothing is typed in the field, an error message should appear and the product should not be saved.

I know this is pretty basic stuff, but I really cannot figure this one out..
Hope someone can help me.

I´m using VM 2.0.6