Store Client Session

When user initiates checkout, fire below event

window._mp.safeExecute('evStoreClientSession', function() {
   eedl('store_client_session', {
      cart_token: '<cart id or cart token>'
    });
  })();

If the event occurs in a iframe window, you need to fire the event using the below script.

window.top.postMessage({
      event_name: 'store_client_session',
      event_payload: {
            cart_token: '<cart id or cart token>'
          }
    },
    'PARENT_URL'
);

PARENT_URL is the url of the website.