A basic example of Custom HTML Tag. GTM

customTag.html

<script type="text/javascript">
(function() {
    var eventsNames = [];

    for (var i = dataLayer.length - 1; i >= 0; i--) {
        eventsNames.push(dataLayer[i].event);
    }


    dataLayer.push({
        'event': 'query',
        'eventsNames': eventsNames,
        'value': 'java-code'
    });

})();
</script>>