Ooof - if the application is not accessible in the sense of being navigable by keyboard alone, then yes, you will need to send mouse clicks (or window messages of the type WM_CLICK).

If you can find the control ID of the button (using the spy.pl program), you can make your click-sender somewhat more resilient by sending the window message to that control, otherwise you need to send it using x/y coordinates and need to hope that nothing changes the layout of the window (font choice, font size, zoom/scaling settings, ...)