Implement your Google Tag Manager container ID on your landing page. Implementation guide available here.
Paste the following code in Javascript - Header section.
<script type="text/javascript">
window.instapageFormSubmitSuccess = function( form ){
window.dataLayer = window.dataLayer || [];
window.dataLayer.push({
'event' : 'formSubmissionSuccess',
'formId' : 'contactForm'
});
};
</script>
With this code snippet on your landing page, you will be able to create custom events and triggers within Google Tag Manager that fire upon a form submission.
