You do not need to. Win64 has only one (1) calling convention (four register _fastcall) and that is selected by default for that platform.
If you want you can select stdcall or winapi on linux too, I seem to remember stdcall on 64 bit is aliased to 4 register __fastcall, but imho makes not much sense, unless, indeed, it is for some of your own routines. So try stdcall on Linux. (which shouldn't be really stdcall,but the aliased __fastcall64). I am not absolutely sure, though.
And may be FastCall simply works.