Use featured image url as post url

If you mean the accompanying permalink for a given attachment, since attachments are posts in a post type themselves, then I think the following snippet would work for you. It’s not something available as a setting for CPTUI but I think this covers your need.

function cptui_featured_img_permalink_as_post_type_permalink( $post_link, $post ) {
	// Only published posts
	if ( 'publish' !== $post->post_status ) {
		return $post_link;
	}

	// CHANGE THIS TO MATCH
	if ( 'movie' !== $post->post_type ) {
		return $post_link;
	}

	// Only if we have a post thumbnail
	if ( has_post_thumbnail() ) {
		// We'll grab the thumbnail ID on the current post, and then grab that thumbnail's permalink and return that.
		return get_permalink( get_post_thumbnail_id( get_the_ID() ) );
	}

	// Otherwise return the original value.
	return $post_link;
}
add_filter( 'post_type_link', 'cptui_featured_img_permalink_as_post_type_permalink', 10, 2 );

You’ll want to change the post type in the snippet to match yours. I used “movie” as a quick demo.

Thank you very much for your fast reply!
Your code works, but I think my question was a bit confusing: I did not mean the attachment page but the image url itsef. So instead of linking to “mywebsite.com/?attachment_id=10356” it need to have a link like “mywebsite.com/redshirt.jpg” while redshirt.jpg is the featured image. Is this possible?
Thank you in advance,
kind regards

Try this version

function cptui_featured_img_permalink_as_post_type_permalink( $post_link, $post ) {
	// Only published posts
	if ( 'publish' !== $post->post_status ) {
		return $post_link;
	}

	// CHANGE THIS TO MATCH
	if ( 'movie' !== $post->post_type ) {
		return $post_link;
	}

	// Only if we have a post thumbnail
	if ( has_post_thumbnail() ) {
		// We'll grab the thumbnail ID on the current post, and then grab that thumbnail's image src at full size and return that.
		return wp_get_attachment_image_url( get_post_thumbnail_id( get_the_ID() ), 'full' );
	}

	// Otherwise return the original value.
	return $post_link;
}
add_filter( 'post_type_link', 'cptui_featured_img_permalink_as_post_type_permalink', 10, 2 );

You’re genius!!! Works like a charm! Thank you so much!!

Welcome