it’s ClassOfAsset'/Path/To/Package.NameofAsset'
, path is in UE4 asset registry (kind of virtual file system) so you should not include uasset file exctation in there. Package in UE4 is file containing asset since they can be packed to single file (via C++), this was standard feature in UE3 and older, in UE4 single asset is packed to single package which is uasset file, so you usally just repeat the name on asset name.
But you dont really dont need to think about it at all, In Content Browser you can right click any asset and there option Copy Reference Path and path will be copy to clipboard will work in C++ functions.