Where is the primary fill hole located on 2008 heritage softail?

On all Harley Big Twins from 1970-up that use a sealed chaincase, primary oil is added by removing the derby cover, also known as the clutch inspection cover. On your bike it's the large round cover with five screws around its perimeter.

The factory recommends replacing the derby cover gasket whenever that cover is removed, but you may find it's not necessary.