Short Code in text area does not work

I have exactly the same issue as this post:
https://www.ads-software.com/support/topic/php-code-in-ot-frontend-textarea/

When I put a shortcode to bring in a Modal link, OptionTree just shows the shortcode and does not excute the function.

Then only answer I can find for this is:
<?php echo do_shortcode( $textarea );?>

But where do I put this? On the page template? In the general functions.php file?

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