и в чём задача?

так вы его и получаете. зачем вам этот хэндл? что вы хотите дальше с ним делать?

API Explanation
GetActiveWindow returns a handle to your program's currently active window. This only works with windows created by your application -- in other words, it won't find the active window of other programs. If your program is in the background, the function will get the window that would be active if the program were active. If an error occurs, or if there is no active window to your program, the function instead returns 0.