Why do I get an indent between two objects after adding a Subsurf modifier? This is my first post. I hope you can help.

I have an object with an extruded face, that is slightly smaller than the width/height of the main object. When adding the subsurface modifier, at the join I am getting a indent between the two objects.

I have tried extra loop cuts, cut it either distorts the extruded parts shape, or creates a strange hard line.
It's just bad topology, you have ngons that are stretched by the Subdivision Surface modifier, use the knife tool and create edges on the top to only keep quads: