OptinMonster Custom Smart Tag to capture a specific query arg.

om-custom-smart-tag-specific-query-args.js

const queryString = window.location.search; // capture all query args
const urlParams = new URLSearchParams(queryString); // parse parameters
const paramName = 'utm_medium'; // create variable to re-use
const medium = urlParams.get(paramName); // call our specific parameter

document.addEventListener('om.Dtr.init', function(event) {
	// Registers {{query_arg}} smart tag that outputs a specific query arg from the current page.
	event.detail.Dtr.setCustomVariable('query_arg', '?' + paramName + '=' + medium);
});