How to access steam API in My Created project CPP file

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Hi guys,
I created a game project with UE4.18 Editor(Not code base engine but run with Epic Games Launcher). For some reason that I must use the Editor Launcher instead of code base Engine.

And then the steps are:
      1: I create a new CPP class(named like SteamAPIHelper) inherited from Actor.h(Just right click in editor content blank space and select "new C++ class"). 
      2:Then get into project solution with VS2015. 
      3:Add  #include "ThirdParty/Steamworks/Steamv139/sdk/public/steam/steam_api.h" in my "SteamAPIHelper.h" file.
 The problem is : I can not get my project compiled success. with some logs like :


         \SteamAPIHelper.gen.cpp(63): error C2059: syntax error: ','
         \SteamAPIHelper.gen.cpp(65): error C2143: syntax error: missing ';' before '}'
         \SteamAPIHelper.gen.cpp(66): error C2065: 'EnumParams': undeclared identifier
         \SteamAPIHelper.gen.cpp(68): error C2059: syntax error: 'return'
         \SteamAPIHelper.gen.cpp(69): error C2059: syntax error: '}'
         \SteamAPIHelper.gen.cpp(69): error C2143: syntax error: missing ';' before '}'

And I want you know that:

    Before I add  "#include "ThirdParty/Steamworks/Steamv139/sdk/public/steam/steam_api.h"" in my file, I can get my project run success, and could upload Steam Leaderboard with the API provided by UE4 Blueprint "Write Leaderboard Integer". what means that I have setup some necessary settings  like in *.ini files or the SteamWorks server.

    Does anyone can do me any help? I just want to Access SteamAPI in my CPP class as UE4 editor don't support to get a list of Steam Leaderboard for now.

Any help would be much appreciated :).
Thanks all.