Voluum and GTM Events Tracking Integration JavaScript Code. This code will send a add_to_cart, begin_checkout and purchase events to your Voluum tracker. Make sure your Voluum events have appropriate names.

voluum-gtm-tracking.html

<!-- page view -->
<script>
	// init user data
	(function () {
		try {
			var url = new URL(window.document.location.href),
			utmMedium = url.searchParams.get('utm_medium'),
			xoClickId = url.searchParams.get('utm_term');
			if (utmMedium === 'xo' && xoClickId) {
				localStorage.setItem('utmMedium', utmMedium);
				localStorage.setItem('xoClickId', xoClickId);
			}
		} catch(e) {
			console.log(e);
		}
	})();
</script>
<!-- page view/ -->

<!-- add to cart -->
<script>
	(function () {
		function getPostbackURL() {
			try {
				var utmMedium = localStorage.getItem('utmMedium');
				var xoClickId = localStorage.getItem('xoClickId');
				if (utmMedium === 'xo' && xoClickId) {
					return 'https://your-voluum.domain/postback?cid='+xoClickId;
				}
			} catch(e) {
				console.log(e);
			}
		}
		if (getPostbackURL()) {
			navigator.sendBeacon(getPostbackURL()+'&et=add_to_cart');
		}
	})();
</script>
<!-- add to cart/ -->

<!-- begin checkout -->
<script>
	(function () {
		function getPostbackURL() {
			try {
				var utmMedium = localStorage.getItem('utmMedium');
				var xoClickId = localStorage.getItem('xoClickId');
				if (utmMedium === 'xo' && xoClickId) {
					return 'https://your-voluum.domain/postback?cid='+xoClickId;
				}
			} catch(e) {
				console.log(e);
			}
		}
		if (getPostbackURL()) {
			navigator.sendBeacon(getPostbackURL()+'&et=begin_checkout');
		}
	})();
</script>
<!-- begin checkout/ -->

<!-- purchase -->
<script>
	(function () {
		function getPostbackURL() {
			try {
				var utmMedium = localStorage.getItem('utmMedium');
				var xoClickId = localStorage.getItem('xoClickId');
				if (utmMedium === 'xo' && xoClickId) {
					return 'https://your-voluum.domain/postback?cid='+xoClickId;
				}
			} catch(e) {
				console.log(e);
			}
		}
		if (getPostbackURL()) {
			var url = getPostbackURL()+'&et=purchase';

			try {
        // use your value or GTM {variable} for "price"
				var price = document.querySelector('.__total_price').innerText;
				price = parseFloat(price.replace(/[^\d.-]/g, ''));
				url += '&payout='+price;
			} catch(e) {
				console.log('no price');
			}

			navigator.sendBeacon(url);
		}
	})();
</script>
<!-- purchase/ -->

Read full article about Voluum+GTM postbacks integration before using this code.