1. The harder way: Trace through all the code until you've found the part, which creates the key. Read it completely until you've understood it and create then a keygen or create a program which bruteforces all possible passwords with this algorithm.
2. The easier way: Put a single breakpoint and look what the application is doing with the key you've entered. Look at the stack or the CPU and enter the string the program expects.
Just patching the app like you did doesn't work here, because the app constructs the final key with an algorithm instead of reading it from some strings in the file.
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