How do I cite the keynote from a Twitter conference?

Since a Twitter conference keynote will likely span several tweets, cite it the way you would cite a Twitter thread. Follow the MLA format template. List the author of the keynote, the text of the first tweet in the thread (shortened if necessary), Twitter as the title of the container, the date, and the URL:

@roopikarisam (Roopika Risam). “Thank you, @annetiquate & @caitduffy49 for the opportunity to speak today and to all of you who are participating. . . .” Twitter, 18 July 2019,

You could also use a description in place of the tweet text:

@roopikarisam (Roopika Risam). Keynote for #HCTwitterConf19. Twitter, 18 July 2019,

If the keynote is available on a site that collects Twitter threads, such as Thread Reader, list that site as the container and the URL for the keynote:

@roopikarisam (Roopika Risam). Keynote for #HCTwitterConf19. Thread Reader, 18 July 2019,