UpdateLayeredWindow

[DllImport("user32.dll", SetLastError = true)]
public static extern bool UpdateLayeredWindow(
   IntPtr hWnd,
   IntPtr hdcDst,
   ref POINT pptDst,
   ref SIZE psize,
   IntPtr hdcSrc,
   ref POINT pptSrc,
   int crKey,
   ref BLENDFUNCTION pblend,
   uint dwFlags
);

Microsoft documentation: Link