@HelloUser000 said:

Here is what I tried {"id": 12}.

You need three key-value pair element,

{
   media_type: 'movie', 
   media_id: 11, 
   watchlist: true
}

To get request JSON body follow this steps:

1. Go to (click this)
2. On right side, select your perefered language programming
3. On dropdown REQUEST EXAMPLE just click REQUEST EXAMPLE
4. For example I choose Node and using FETCH REQUEST, the output on the box area will be

const url = 'https://api.themoviedb.org/3/account/21457174/watchlist';
const options = {
  method: 'POST',
  headers: {
    accept: 'application/json',
    'content-type': 'application/json',
    Authorization: 'Bearer {{YOUR_API_READ_ACCESS_TOKEN}}'
  },
  body: JSON.stringify({media_type: 'movie', media_id: 11, watchlist: true})
};
fetch(url, options)
  .then(res => res.json())
  .then(json => console.log(json))
  .catch(err => console.error(err));

you can see the json body is:

...
body: JSON.stringify({media_type: 'movie', media_id: 11, watchlist: true})
...

Thanks