code: function(playlistId, tracks, options, callback) {
      return WebApiRequest.builder(this.getAccessToken())
        .withPath('/v1/playlists/' + playlistId + '/tracks')
        .withHeaders({ 'Content-Type': 'application/json' })
        .withQueryParameters(options)
        .withBodyParameters({
          uris: tracks
        })
        .build()
        .execute(HttpManager.post, callback);
    }
docs: Add tracks to a playlist.
  @param {string} playlistId The playlist's ID
  @param {string[]} tracks URIs of the tracks to add to the playlist.
  @param {Object} [options] Options, position being the only one.
  @param {requestCallback} [callback] Optional callback method to be called instead of the promise.
  @example addTracksToPlaylist('thelinmichael', '3EsfV6XzCHU8SPNdbnFogK',
  '["spotify:track:4iV5W9uYEdYUVa79Axb7Rh", "spotify:track:1301WleyT98MSxVHPZCA6M"]').then(...)
  @returns {Promise|undefined} A promise that if successful returns an object containing a snapshot_id. If rejected,
  it contains an error object. Not returned if a callback is given.