How do I test a function that sends a request in browser that requires authentication?

I'm in the process of writing unit tests in Mocha and Chai, and I'm trying to figure out how to test a function that that uses the fetch API to send a request that requires a session to access. This function is an ES6 method that returns a thenable object.

_auth(record) {
    var authMetadata = this._getAuthMetadata(record);
    var authUrl = `/${this._getPortal(window.location.pathname)}/tlist_child_auth.html?${this._encodeUri(authMetadata)}`;
    if (getPortal() !== 'guardian') {
      authUrl += `&frn=${this.coreTableNumber}${this.foreignKey}`;
    }
    return fetch(authUrl, {
      credentials: 'include'
    }).then(function(rawData) {
      return rawData.text();
    });
  }

tlist_child_auth.html is the page that requires session authorization to access. Is there any easy way to do this, or should I seek to "invent the wheel"?