Customize the HTML of a link field from a custom theme
Try this:

{% for item in items %}
  &lt;a href="{{ item.content['#url'] }}" class="btn-circle btn-circle--blue js-open-modal"&gt;
    {{ item.content.text }}
  &lt;/a&gt;
{% endfor %}


Found it in a video from 2015, where MortenDK is showing off
It's about 25 min. in

DrupalCon Los Angeles 2015: Drupal 8 Theming with Love