ImVec2 pos;
ImGui::PushStyleVar(ImGuiStyleVar_Alpha, 1.f);
ImGui::SetNextWindowSize(ImVec2(horizontal size, vertical size));
if (ImGui::Begin("Watermark", NULL, ImGuiWindowFlags_NoTitleBar | ImGuiWindowFlags_NoResize | ImGuiWindowFlags_NoCollapse | ImGuiWindowFlags_NoScrollbar | ImGuiWindowFlags_NoBackground))
{
auto Render = ImGui::GetWindowDrawList();
pos = ImGui::GetWindowPos();
// example
Render->AddRectFilled(ImVec2(pos.x, pos.y), ImVec2(pos.x + 37.5, pos.y + 28), ImColor(0, 0, 0, 230));
}
ImGui::End();
ImGui::PopStyleVar();