Add To Watchlist api not working

I didn't understand the body parameters what is should be???

const options = {
  method: 'POST',
  headers: {
    accept: 'application/json',
    'content-type': 'application/json',
    Authorization: 'Bearer XXXX'
  },
  body: JSON.stringify('{account_id:20394161}')
};

fetch('https://api.themoviedb.org/3/account/20394161/watchlist', options)
  .then(response => response.json())
  .then(response => console.log(response))
  .catch(err => console.error(err));