The base project URL will accept a POST param called number. Its response will be a status of "low", "high", "correct", or "out of bounds". Based on those responses, your task is figuring out what number the server is thinking of. You're more than welcome to use Stack Overflow/the internet and the AJAX library of your choice.
Once you've guessed the correct number, the payload will also include a property called nextNumber. The number will be higher than 2000, but if it's "guessed" using the format from step 1, it will return an object with both letter and nextNumber properties. By assembling the letters over the course of guessing each nextNumber, you'll put together a sentence, which is the solution to the puzzle. (The final letter will not have a nextNumber included.)