According to Google's recommendation, the urls of a sitemaps.xml file must be included in the robots.txt file.

If this is properly included, you will have no extra configuration to do. OnCrawl will automatically take into account your files.

If the urls contained in the files are not listed in the robots.txt, you must specify these urls in the Crawl Configuration.

Here are the steps to follow:

List your sitemaps.xml urls

Edit (or create) your crawl configuration by clicking on “Launch crawl” on your project homepage

In the crawl setting, find the “sitemap” section, check the “Specify sitemaps URLs” box and add all your sitemap or sitemap index urls.

Then save and launch the crawl.

When the crawl will be ready you will find the detailed report on the “Crawl Report>Indexability>sitemaps” section.

This report is enabled by default, but you can disable it when setting up a crawl. To disable it, uncheck the "Enable sitemaps cross-data analysis" under "Sitemaps" in your crawl settings.

When disabled, you won't see the "Sitemaps" dashboard in your crawl.