https://twitter.com/AnnieCushing/status/806518054368739329

answer.js

/**
 * To answer @AnnieCushing's question on Twitter:
 * https://twitter.com/AnnieCushing/status/806518054368739329
 * 
 * Author: Chris Le - @iamchrisle
 *
 * There's a few ways to do this.
 * - Like @SimoAhava suggested, server redirects that strips parameters if you
 *   have the ability to control the server.
 *  
 * Here's a JavaScript / GTM solution if you only have the ability to change
 * the code on the pages: Use the DataLayer to hold either the 
 * UTM campaigns if the referrer is or is not "primarydomain.com".
 *
 * This code would go on every page near the GTM tag.
 *
 * Technically speaking if you have nothing but access to GTM and GA, you could
 * also sneak this code into a custom HTML tag.
 *
 * So.. this would go into the page...
 */

(function() {
  var getParameterByName = function(name, url) {
    if (!url) {
      url = window.location.href;
    }
    name = name.replace(/[\[\]]/g, "\\$&");
    var regex = new RegExp("[?&]" + name + "(=([^&#]*)|&|#|$)"),
        results = regex.exec(url);
    if (!results) return null;
    if (!results[2]) return '';
    return decodeURIComponent(results[2].replace(/\+/g, " "));
  }

  if (document.referrer.indexOf('primarydomain.com') == -1) {
    window['dataLayer'].push({
      utmVariables: { 
        campaignSource: getParameterByName('utm_source'),
        campaignCampaign: getParameterByName('utm_campaign'),
        campaignMedium: getParameterByName('utm_medium'),
        campaignTerm: getParameterByName('utm_term'),
        campaignContent: getParameterByName('utm_content')
      }
    });
  } else {
    window['dataLayer'].push({
      utmVariables: {
        campaignSource: '',
        campaignCampaign: '',
        campaignMedium: '',
        campaignTerm: '',
        campaignContent: ''
      }
    });
  }
})();

//////////////////////////////////////////////////////

/**
 * Then in GTM create a few dataLayer variables that pulls that stuff.
 *
 * One variable named "UTM Source"
 *   Data Layer variable name: "utmVariables.campaignSource"
 *   Data Layer version: 2
 *   
 * One variable named "UTM Campaign"
 *   Data Layer variable name: "utmVariables.campaignName"
 *   Data Layer version: 2
 *
 * ... and so on for all 5 variables.
 */

/**
 * Finally, in your Google Analytics tag, under "more settings", "fields to set"
 * add them:
 *
 *  +-----------------+--------------------+
 *  | Field name      | Value              |
 *  +-----------------+--------------------+
 *  | campaignSource  | {{ UTM Source }}   |
 *  | campaignName    | {{ UTM Campaign }} |
 *  | ..... so on.    |                    |
 *  +-----------------+--------------------+
 */