I would read this article, its from Yoast. I don’t think you are understanding what the focus keyword feature does. This article talks about that as well as how adding the keyword meta tag is a waste of time (in most cases).
I wouldn’t worry about adding it BUT if you still want to I would add a custom field. This is easiest done with ACF. You can easily create a text field and have it show on every page. Then in your header.php just add something like this…
<meta name="keywords" content="<?php the_field('my_field'); ?>">
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