Open the Window Family, delete the Opening Void and replace it with a Void Extrusion drawn in Plan View, You can draw a non-rectangular sketch boundary that way so the Void won't cut that portion of the Wall. You can make the Void Cut fully parametric as well.
..actually, I may have misunderstood you. I think you may be referring to Wall Closure/Wall Wrapping at Inserts.
Revit Wall Wrap Closure - A How To Guide | Revit Products | Autodesk Knowledge Network
Revit Wall Closers – Layer Wrapping at Wall Inserts & Ends – 8020 BIM
"Wall Closure" is a Reference Plane Properties. Wall Wrapping is a Wall Property.