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, twitter.com/roopikarisam/status/1151919685149036545.
You could also use a description in place of the tweet text:
@roopikarisam (Roopika Risam).
Published 19 July 2019