Social Sharing / tell a friend plug but specify which URL to share

I would recommend rolling your own. Use the sharethis api or addthis. Then you can use get_permalink(); for the url parameter.

function wpu_share() {
        $url = get_permalink();
        $title = get_the_title();
        $add_this="st_url="".esc_url( $url ).'"'; ?>

    <span class="st_facebook_large" <?php echo $add_this; ?> displayText="Facebook"></span>
    <span class="st_twitter_large" <?php echo $add_this; ?>  displayText="Tweet"></span>
    <span class="st_linkedin_large" <?php echo $add_this; ?> displayText="LinkedIn"></span>
    <span class="st_email_large" <?php echo $add_this; ?> displayText="Email"></span>
    <?php }

 add_action('wp_footer', 'wpu_share_js' );
 function wpu_share_js() { ?>
        <script type="text/javascript">var switchTo5x = true;</script>
        <script type="text/javascript" src="http://w.sharethis.com/button/buttons.js"></script>
       <script type="text/javascript">stLight.options({publisher:"ur-xxxxxxxxxxxx"}); </script>
<? }

Update

You will need to pass a variable to the thank you page that references the post id of the course they signed up for then you can use this to get the right url.

$url = get_permalink( $post_id );  

// You will need to pass the post id to the thank you page using a query string or another method.

To use a query string append ?id=$post->ID to the end of the redirect to the thank you page. Then on the thank you page:

if (isset( $_GET['id'] ) $post_id = (int) $_GET['id'];

For security make sure you use esc_url() where ever you are using the $url variable.