REVERSE ENGINEERING | figure out how things work, then break those things | HARD
This flag is about knowing a concept such as brute-forcing (see https://en.wikipedia.org/wiki/Brute-force_attack). The first part is that you have to understand how to send an unlock code. The second is to speed up the process of breaking the code.
Hints
ChatGPT is pretty good at writing Python code. If you ask it to write you a program that attempts all possible combinations and sends them to that URL while printing the result, you will probably get a working solution.
Be careful: the endpoint may have rate-limiting enable to slow down other types of attacks. You may also have to slow your code down just a little to deal with this.
Now you just have to find a place to run your Python code.