Lets go with an example…
Custom Field Name: buy-ticket
Custom Field Value: http://example.com/link-to-ticket/
And you said, you want a button called “Buy ticket” linking to the Custom Field Value. This how you do it:
<a href="https://wordpress.stackexchange.com/questions/57443/<?php echo get_post_meta($post->ID,"buy-ticket', true); ?>" title="Buy a ticket right away!">
<span>Button text or code here</span>
</a>
SOURCE: WordPress Codex Function Reference/get_post_meta
PS: You can disregard the <span>
tag. I used it for cosmetic purposes. 🙂