If you know the window to send them to, you can use WM_MOUSEMOVE and WM_MOUSEACTIVATE. But you have to know the HWND. One way to do that is to use EnumWindows and EnumChildWindows to find the button in the app you are looking for.
Another approach is to pick an x,y and call WindowFromPoint() or ChildWindowFromPoint, and then with the HWND that returns, send IT the WM_MOUSEMOVE or WM_MOUSEACTIVATE.
Hope that gives you a starting point.