/admin/users/sync_sso ... Ruta no encontrada

Code that produces the bottom query:

const userparams = {
      external_id: user._id.toString(),
      email: user.email,
      username: user.username.replace(/\s/g, ''),
    };
    const payload = new Buffer(querystring.stringify(userparams), 'utf8').toString('base64');
    const hmac = crypto.createHmac('sha256', DiscourseSSOKey);
    hmac.update(payload);
    const query = querystring.stringify({
      sso: payload,
      sig: hmac.digest('hex'),
      api_username: UserName,
      api_key: API Key,
    });
    console.log(query);
    fetch('https://community.somesite.com/admin/users/sync_sso', {
      method: 'POST',
      body: query,
    })
      .then(res => res.json())
      .then(json => console.log(json));    

Here’s the query that gets generated and passed as the body on the POST route:

sso=somelongSSOgenerated.&sig=somelongsignature&api_username=someusername&api_key=someAPIKey

This all still gives me a Route Not Found error in the rails logs.