Make a private API request
function (command, parameters, callback) {
      if (typeof parameters === 'function') {
        callback = parameters;
        parameters = {};
      }

      parameters || (parameters = {});
      parameters.command = command;
      parameters.nonce = nonce();

      const options = {
        method: 'POST',
        url: PRIVATE_API_URL,
        form: parameters,
        headers: this._getPrivateHeaders(parameters)
      };

      return this._request(options, callback);
    }