It is not required to have the Sitemap XML link in your robots.txt
There are 3 methods of getting search engines aware of your sitemap XML files:
The recommended way is submitting the link manually. Please check this article:
Also, on the Sitemap XML configuration page there are two checkboxes that will make XC5 inform Google and Bing servers about new sitemaps when you generate them (this is the third method from the link to sitemaps.org site above).
You may also edit your robots.txt and add the link manually.