Discord blocks Roblox requests. I don’t exactly know why, but don’t go for any work arounds like using another website’s webhook, since using their’s can be dangerous. There is no way to get communication from your game to you unless you somehow make your own secure website which can take a while to learn.
Are you sure that the string in the “content” parameter is not empty? Discord doesn’t allow empty messages to be sent.
And 400 means bad request which is a status code to tell that the data you provided is invalid and not that you do not have authorization, in the second case it would either be 401 or 403