unable to FillRect with transparent brush when double buffering - c++

Question

As the title says I'm unable to FillRect bitmap to be transparent. I know when creating the bitmap it is not monochrome as gray brush works fine but I have no way (that I'm aware of) to check if it is colored or grayscale. I also am aware that by default the bitmap is black hence why I'm trying to change it to transparent. I am also aware that I'm likely not cleaning up the dc's correctly however that is not the main issue. I'm trying to solve the black background by making it transparent.
#include<windows.h>
#include<iostream>
int main()
{
// Init DC
HWND Wnd = GetDesktopWindow();//GetConsoleWindow();
HDC ScreenDC = GetDC(Wnd);
// Init Rectangle
RECT ClientRect;
GetClientRect(Wnd, &ClientRect);
// Init Double Buffer
HDC MemDC = CreateCompatibleDC(ScreenDC);
HBITMAP MemBM = CreateCompatibleBitmap(ScreenDC, ClientRect.right - ClientRect.left, ClientRect.bottom - ClientRect.top);
HBITMAP OldBM = (HBITMAP)SelectObject(MemDC, MemBM);
// Create Brush and Pen
HPEN Pen = CreatePen(PS_SOLID, 1, RGB(255, 0, 0));
HBRUSH ClearBrush = (HBRUSH)GetStockObject(GRAY_BRUSH);
// Set Brush and Pen
SelectObject(MemDC, Pen);
SelectObject(MemDC, ClearBrush);
POINT p;
while(!GetAsyncKeyState(VK_RETURN))
{
// Clear and Draw
GetCursorPos(&p);
FillRect(MemDC, &ClientRect, ClearBrush);
Rectangle(MemDC, p.x, p.y, p.x+20, p.y+20);
BitBlt(ScreenDC, 0, 0, ClientRect.right - ClientRect.left, ClientRect.bottom + ClientRect.left, MemDC, 0, 0, SRCCOPY);
}
SelectObject(MemDC, OldBM);
DeleteObject(ClearBrush);
DeleteObject(Pen);
DeleteObject(OldBM);
DeleteObject(MemBM);
DeleteDC(MemDC);
ReleaseDC(Wnd, ScreenDC);
return 0;
}
I've tried many different ways of setting transparent background to no avail. The end result is a rectangle appearing over the mouse and following it across the screen however the background shouldn't be black I should be able to see other windows.