Google Tag Manager form submission
<script type="text/javascript">
window.instapageFormSubmitSuccess = function( form ){
window.dataLayer = window.dataLayer || [];
window.dataLayer.push({
'event' : 'formSubmissionSuccess',
'formId' : 'contactForm'
});
};
</script>
custom event GTM: http://take.ms/7hrXh
GTM tag: http://take.ms/aAzKJ