The app has a "secret dialog window" and I'm sure it appears after pressing combination of some buttons. How I can find the combination in this app?
I assume that you don't have any more information about this Secret dialog window than what you have written (that is you don't know how it looks like and the strings it contains, etc.).
In such a case, you can search for GetKeyboardState function that may be used to obtain the keys pressed at a given time. After finding it, just look at the subsequent functions calls and you will probably find the function responsible for handling the information provided by
GetKeyboardState function, thus checking whether the keys being pressed match one of the program keyboard shortcuts.
As you may notice, it firstly converts it to
ASCII code and then performs some checks.