Quick hook into the Twenty Fifteen Reply to link

after-reply-link.php

<?php
function ck_our_reply_additions( $text, $args, $comment, $post ) {
	$text = $text . ' <button>Click here to like this comment</button>';
	return $text;
}
add_filter( 'comment_reply_link', 'ck_our_reply_additions', 10, 4 );