Follow the MLA format template. Say, for example, you wish to cite a marriage index on Ancestry as your source for the date of a person’s marriage. List the title of the index as the title of the source, Ancestry as the container, the copyright date of the site (since no publication date is given for the marriage index), and the URL where the index is located. As always, key your in-text citation to the first element of the entry:

Sylvia Stermer married Arthur Lachar in New York City on 25 February 1943 (New York City).

Work Cited

New York City, Marriage Indexes, 1907-1995. Ancestry, 1997-2018, www.ancestry.com/family-tree/person/tree/71574598/
person/180149921221/facts.

If you cite more than one marriage from the index, insert an endnote at first instance explaining that all the marriages cited in the text correspond to the works-cited-list entry beginning “New York City.”