user/channel/broadcaster id are all the same thing, they are just named differently depending on context.
If you’re sure the id is correct, then the only other thing I can suggest is you double check your URL that you’re posting to.
I’m not sure if the code you pasted here lost some syntax or formatting in pasting, but url: ‘https://api.twitch.tv/helix/channel_points/custom_rewards?broadcaster_id={broadcaster_id}’ will send the post to that exact URL, you need to add your id to the broadcaster_id param, because based on what you’ve pasted you’re literally sending the text {broadcaster_id}, rather than 123456 or whatever your id is.