matomo/plugins/CustomDimensions/templates/_actionTooltip.twig

{% if action.customDimensions is defined %}
    {% for dimension,value in action.customDimensions|filter(value => value|length > 0) %}
        {% if loop.index == 1 %}

            {{ 'CustomDimensions_CustomDimensions'|translate }}:
        {%- endif %}

        {# line break above is important #}
        - {{ dimension|raw }} = {{ value|raw }}
    {%- endfor -%}
{%- endif -%}