How to add Facebook like button to upPrev box?

You can use iworks_upprev_box_item filter, to modify item element.

<?php
add_filter( 'iworks_upprev_box_item', 'iworks_upprev_box_facebook_like' );
function iworks_upprev_box_facebook_like($content)
{
    /**
     * change this for your data
     */
    $appId = 279100175534801;
    $like_page = site_url();
 
    ob_start();
?>
<hr />
<div id="fb-root"&</div>
<script>(function(d, s, id) {
  var js, fjs = d.getElementsByTagName(s)[0];
  if (d.getElementById(id)) return;
  js = d.createElement(s); js.id = id;
  js.src = "//connect.facebook.net/<?php echo get_locale(); ?>/all.js#xfbml=1&appId=<?php echo $appId; ?>";
  fjs.parentNode.insertBefore(js, fjs);
}(document, 'script', 'facebook-jssdk'));</>
<div class="fb-like" data-href="<?php echo $like_page; ?>" data-layout="standard" data-action="like" data-show-faces="false" data-share="false"></div>
<?php
    $content .= ob_get_contents();
    ob_end_clean();
    return $content;
}

Get code: upprev_add_fb_like_to_box.php

Where to put this code?

There are three ways:

  1. made a one-file-plugin and put file into wp-content/plugins, then turn it on
  2. made a one-file-plugin and put file into wp-content/mu-plugins
  3. put this code into functions.php in current theme

I recommend using option 1 or 2, but 3 is the easiest.