This is snippet that we've used for ages on our site so I thought I'd share it with you. What it enables you to do is add an external link to the featured image on the main post page. This is particularly useful for linking to client websites, affiliate links or any manner of URLs where linking out to an alternate URL is desirable.
WordPress add External link to Featured Image
Updated for Custom Fields with Gutenburg WordPress 5.0 (see step 2 on how to enable Custom Fields for Gutenburg)Just 3 steps
1. Add the following WordPress snippet to your Wordpress Theme functions.php file
Save your updated functions.php file
2. Ensure that you have WordPress custom fields enabled in your version of WordPress (previously in top admin dropdown). If you are using WordPress 5.0 with Gutenburg and Custom Fields are not visible then click on the 3 vertical button burger menu (top right), head to 'Options' and ensure they are ticked under 'Advanced Fields'.
3. Now compose your post as usual and add the featured image. Then access the Custom Fields panel on your post page and 'Add New Custom Field' ExternalUrl and then add your desired destination external URL in the 'Value' entry like the below and that's it - your featured image will now link externally to the Link set.
We now use Webflow as this sort of feature is much easier on this superior CMS and we encourage you to take a look. If you stick with WordPress then we hope you find this snippet useful?