gistfile1.php

<?php
  if(tribe_get_custom_field('eventtype') == 'Webinar') {
?>

<input type="button" value="Click here to go somewhere" onclick="window.open('<?php echo tribe_get_custom_field('WebinarURLField'); ?>');" />

<?php 
  } elseif(tribe_get_custom_field('eventtype') == 'Lunch') {
?>

<input type="button" value="Click here to go somewhere" onclick="window.open('<?php echo tribe_get_custom_field('LunchURLField'); ?>');" />

<?php } ?>