Display custom field value into content with hyperlink

I think this is what you’re looking for…

function wpa_content_filter( $content ) {
    global $post;
    if( $meta = get_post_meta( $post->ID, 'demo_link', true ) ) {
        return $content.' <a href="'.$meta.'">'.$meta.'</a>';
    }
    return $content;
}
add_filter( 'the_content', 'wpa_content_filter', 10 );