custom field output after the_content

This is the answer – code to be added in the functions.php:

add_filter( 'the_content', 'my_the_content_filter', 0 );
function my_the_content_filter( $content ) {
if ( is_single() )
{
    global $post;
    $pgLnk=get_post_meta($post->ID, 'Button', true);
    $content .= '<div id="button-link"><a href="'.$pgLnk.'" target="_blank">
    <span class="button-link"></span></a></div>';
}
return $content;
}

This code was wrote by @Sheikh Heera on stackoverflow.com – direct link