How or what message to send to move one window over another?

Question

API

I'm practicing in WinApi and I have a task to implement the movement of one window by moving the pressed cursor in the client area of ​​another window .
In the first window, movement is implemented when the mouse is held down in the client area as follows:

case WM_LBUTTONDOWN:
    {
        flag = true;
        break;
    }
case WM_MOUSEMOVE: {
    if (flag) {
      ReleaseCapture();
      SendMessage(hWnd, WM_NCLBUTTONDOWN, HTCAPTION, 0);
    }
    break;
  }

Is there any way to make this work when sending messages to the first window from the second window? If yes, how to do it?