I’m not sure I’ve understand, but I’m supposing the blueprint derives from the c++ class and you want to set the mesh from the blueprint.
The blueprint doesn’t have to “implement” nothing, because that is just a property, properties are not implemented, methods do.
You will have to create the StaticMesh in the c++ class (with CreateDefaultSubobject) or else the blueprint will crash if it tries to access it.
Do you want do set the 3d model mesh from the blueprint? If that’s the case, just make the property visible/editable in blueprint and then you will have the static mesh properties available. Then you can set the mesh.
The ACharacter class does this.
Of course I might be completely wrong!