[Plugin: WP CSS Button (CTA-Button Plugin)] Buttons in Widgets

Never mind. I found this https://www.smashingmagazine.com/2009/02/02/mastering-wordpress-shortcodes

The problem. Even if you enjoyed this article, you may have felt a bit frustrated because, by default, WordPress doesn’t allow shortcode to be inserted into sidebar widgets. Thankfully, here’s a little trick to enhance WordPress functionality and allow shortcodes to be used in sidebar widgets.

The solution. One more piece of code to paste in your functions.php file:
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1 add_filter('widget_text', 'do_shortcode');