Shortcodes, as I understand them, are to be used inside content of pages/posts, and (with 4.9) widgets. They aren’t supported inside other spots on the page, only in the page/post/widget content.
From the Codex: https://codex.wordpress.org/Shortcode
[WP] only expands the shortcodes within the content of a Post, Page,
or custom post type.
But I don’t think your code should be setting a shortcode. Perhaps it should be setting a global variable that will be used elsewhere. (But it is late, and I am under the influence of excessive turkey…)