How to make Reference relationship control Mandatory on OPA Interview

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I Can't see any Option to make Reference relationship control Mandatory in OPA Interview Tab. for now i created a reference relationship and able to pull loaded OSC Instances to Reference relationship  dropdown and can't able to make it mandatory. I do have one complex scenario, from OSC Object i am Loading Category,Sub-Category,Sub-category-Description as Instances based on Entity Condition.Now,Based on Category,the Dynamic load of Sub-category should happen and again based on Sub-Category the Dynamic Load of Sub-category-Description in Drop-down should happen.For Example,Based on Entity Condition Category instances are 1,2 & Sub-category Instances are A,B,C,D and Sub-category-Description Instances are a,b,c,d. Now