Add Checked Attribute to Custom User Fields on Frontend User Form
Solved!

{% for letter in currentUser.newsletters.options %}
    &lt;input type="checkbox" value="{{ letter.value }}" name="{{ letter.value }}" id="{{ letter.value }}"
        {% if letter.selected == true %}checked="checked" {% endif %}/&gt;
    &lt;label for ="{{ letter.value }}"&gt;{{ letter.label }}&lt;/label&gt;
{% endfor %}