Automatically Setting the First Image in WordPress Articles as Featured Image

Question

I'm using WordPress version 6.4.1 with the twentytwentyfour theme. The images are hosted externally。 Is there a way to achieve this through custom code? I'm open to using functions.php or any other recommended method. Could someone provide guidance or code snippets on how to implement this functionality?

$thumbnail_url = get_post_meta(get_the_ID(), 'thumbnail_url', true);
if ($thumbnail_url) {
    echo '<img src="' . esc_url($thumbnail_url) . '" alt="Custom Thumbnail">';
}

i don't know where to place the code to retrieve and display this custom thumbnail.