flaskbb/flaskbb/templates/email/activate_account.html

{% set link = url_for('auth.activate_account', token=token, _external=True) %}

<p>{% trans user=user.username %}Dear {{ user }},{% endtrans %}</p>

<p>{% trans %}Click the link below to activate your account:{% endtrans %}</p>

<p><a href="{{ link }}">{{ link }}</a></p>

<p>{% trans %}Sincerely,{% endtrans %}</p>
<p>{% trans %}The Administration{% endtrans %}</p>