Hi, you have good reviews so I'm just posting the code here in good faith.
Inside the loop for the event post template, try adding this:
<?php $link = get_post_meta(get_the_ID(), 'tickets', true); ?>
and then modify your code like this:
...
<a href="' . $link . '"
...
If you're happy with the result award me the project later. Let me know if you have any questions - I'll be happy to answer!