Unable build dll

Code
1

LINK : error LNK2001: unresolved external symbol _DllMainCRTStartup

cmake version 3.18.20081302-MSVC_2
and
cmake version 3.16.5

CMakeLists.txt

cmake_minimum_required(VERSION 3.16)
project(testttttt)

include(GenerateExportHeader)

add_library(mylib SHARED Export.h)
generate_export_header(mylib)
set_target_properties(mylib PROPERTIES LINKER_LANG

Export.h

#include <string>

#include "mylib_export.h"

MYLIB_EXPORT int Export(const char* data_c, size_t size){
    std::string http_data(data_c, size);
    return 333;
}
2

You need a source file. Header file by itself will not cut it.