Sub form auto field population based on parent form

Sub form auto field population based on parent form

I have a parent form called "Sites" with a text field called "Site". I have a subform called "Design Comments" (actual form name "Review Comments") with a lookup field name "Sites1" that looks up from the Sites form. I want the Sites1 lookup field to automatically populate with the parent record when a new subform row is added. What is the correct way to achieve this?