I was stuck on this for a looooooooong time because I didn’t fully understand what I was meant to be doing, it’s relatively simple because you have the password already - all you have to do is ‘POST’ it.
You don’t need to download any tool, everything you need is in the source code. Think about how a website would normally ‘POST’ information.. find a way to create a certain object to ‘POST’ the infomation provided. Maybe check a previous level to see how its formulated.
Keep at it, you’ll eventually get it and you’ll feel a huge relief once you’ve sussed it out. Like I said, I was stuck on this for ages so much so it pissed me off royally!