Make relationship filed required

Hi All

I am wandering how I can make a field required, this is a rationship field and cant see it in studio. 

As its not in studio i was going to write a vardef file as add the argument 

'required' => 'true',

the problem is i don't know the field name to then target with a vardef. I know the data is stored in  p_product_s_serialnumber_1 table, field p_product_s_serialnumber_1p_product_ida