Write the full MLA citation for your ad. (It will probaby be a citation of a Youtube video.) Eventually, it will go in the Works Cited section at the end of your paper.
Author of content. "Name of Video." YouTube, Name of uploader, Date of post, URL.
Example using Mrs. Doyle's ad:
Apple. "iPhone 7 Plus--The City." YouTube, Apple, 1 May 2017, https://youtu.be/waTteMeg4Ag.
The author will usually be the company who makes the product being advertised.
The second line should be indented.
The dates are in day, month, year order. Do not abbreviate May, June, and July. Abbreviate September as Sept (without a period after it). Abbreviate all other months to the first three letters. Do not put a period after the abbreviation.