To retrieve a client ID on the source domain, you use the get method:
ga(function(tracker) { var clientId = tracker.get('clientId'); }); destination.com ga('create', 'UA-XXXXX-Y', 'auto', { 'clientId': getClientIdFromUrl() }); window.addEventListener('message', function(event) { // Ignores messages from untrusted domains. if (event.origin != 'https://destination.com') return; ga('create', 'UA-XXXXX-Y', 'auto', { clientId: event.data }); } );// Stores whether or not the tracker has been created. var trackerCreated = false; function createTracker(opt_clientId) { if (!trackerCreated) { var fields = {}; if (opt_clientId) { fields.clientId = opt_clientId; } ga('create', 'UA-XXXXX-Y', 'auto', fields); trackerCreated = true; } } window.addEventListener('message', function(event) { // Ignores messages from untrusted domains. if (event.origin != 'https://destination.com') return; // Cr...