Resume downloading from a certain position - c++

Question

I need to download the file after a pause. But I do not know how to implement it correctly using https://github.com/yhirose/cpp-httplib . But the problem is that when the download resumes, the server starts sending me the file first. Question: how do I tell the server the size of the piece that I have already downloaded, so that the server sends me only the necessary part?
My code:
std::string body;
httplib::Client cli( url, port );
cli.set_follow_location( true );
int file_size = is_part_of_file ? GetFileSize( result_file_name.c_str() ) : 0; // it is downloaded part of the file
int last_percent;
auto res = cli.Get(
file_path.c_str(), httplib::Headers(),
[ & ]( const httplib::Response& response )
{
( void )response;
return *is_download;
},
[ & ]( const char* data, size_t data_length )
{
body.append( data, data_length );
return *is_download;
},
[ & ]( uint64_t len, uint64_t total )
{
int percent = ( int )( len * 100 / total );
if( last_percent != percent )
{
*p_percent = ( ( float )percent / 100 );
}
last_percent = percent;
return *is_download;
} );
if( res )
{
std::ofstream out( result_file_name, std::ios::binary | std::ios::app );
out << body;
out.close();
}
else
{
if( is_part_of_file )
{
std::ofstream out( result_file_name, std::ios::binary | std::ios::app );
out << body;
out.close();
}
return false;
}