WM_MOUSEMOVE message

The WM_MOUSEMOVE Message is posted to a window when the cursor moves. If the mouse is not currently captured, this message is posted to the window that contains the cursor. If the mouse is captured, the WM_MOUSEMOVE message is posted to the window that has captured the mouse. A window receives this message through the windows procedure function.

Use the following code to obtain the horizontal and vertical position:

xPos = GET_X_LPARAM(lParam);
yPos = GET_Y_LPARAM(lParam);

If an application processes this message, it should return zero.

Source: https://docs.microsoft.com/en-us/windows/win32/inputdev/wm-mousemove