Nevermind. I was able to get the functionality I needed.
{% include 'event-popup.twig' with {
'text_venue_separator': text_venue_separator
} %}
<div style="background: inherit;">
{% if event.get( 'ticket_url' ) is not empty %}
<a href="{{ event.get( 'ticket_url' ) | e('html_attr') }}" target="_blank"
class="ai1ec-tickets ai1ec-btn ai1ec-btn-sm ai1ec-btn-primary
ai1ec-tooltip-trigger"
title="Register"
data-placement="left">
<i class="ai1ec-fa ai1ec-fa-ticket ai1ec-fa-fw"></i>
<span>
Register
</span>
</a>
{% endif %}
</div>