Ok, I understand that you want to implement an interface call in C++? The syntax you use is not correct. Check out these links, maybe it will help:
But to me, if it is not a neccessity to implement interface in C++ (for whatever reason), implementing Interface call in BP is easier, at least at first. After you create BP based on C++ class, in class settings of your BP you can choose Interface it implements, then you will have your Interface events in BP, and can implement functionality.