We apologize for the confusion. The Yoast SEO focus keyword is used by the content analysis feature to provide feedback on how to improve the content on your site and is specifically not the same as the meta keywords output in the head tag of HTML.
By default, our plugin doesn’t give you a field to enter meta keywords, as meta keywords have been proven to be mostly a waste of time. Joost wrote an article on meta keywords that covers this topic. If you’ve read that and still would like to enter meta keywords, you can enable them quite easily.