const auto& window = ImGui::FindWindowByName( "main window name" ); // or FindWindowByID( main_window_id );
ImGui::SetNextWindowPos( window->Pos + ImVec2( window->Size.x + ImGui::GetStyle( ).WindowPadding.x, 0.0f ) );
if ( ImGui::Begin( "shmon'ka" ) ) {
//some shit
}
ImGui::End( );