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I cannot change the url of the photo in the featured-image edit section. The page can be found at https://irregularus.com/tips-and-stories/ and if you click on the picture you will see that it just opens the picture.
<?php the_permalink(); ?>
to get the link to the current post. Essentially replace whatever the href property of the image link is right now so it looks similar to :
<a class="post-thumbnail" href="<?php the_permalink(); ?>" >
Required, but never shown