You can add any XML sitemap or sitemap index URL and the tool will crawl it and create two files, one containing all the URLs, and another containing all the sitemaps and the number of URLs each has.
E.g to test you can use: https://www.google.com/sitemap.xml
Limits: This has been tested with sitemaps of up to 14 million URLs and it takes 30 mins. For most sitemaps it should take seconds or minutes.
The tool takes time based on two things, number of sitemaps to analyze, and size in terms of URLs. If you get 0 URLs its most likely that the crawling was blocked.