begin_checkout

window.dataLayer = window.dataLayer || [];
window.dataLayer.push({ ecommerce: null });
window.dataLayer.push({
  event: 'begin_checkout',              // name of the event. In this case, it always must be begin_checkout
  ecommerce: {
    items: [{                           // an array with all products (that were in a cart when the checkout began)
      item_name: 'Product 1',           // insert an actual product name
      item_id: 'product1',              // insert an actual product ID
      price: '11.99',                   // insert an actual product price. Number or a string. Don't include currency code
      affiliation: 'My online shop',    // affiliation (e.g. affiliate id, name of the store, etc.)
      item_brand: 'brand A',            // insert an actual product price
      item_category: 'Apparel',         // insert an actual product top-level category
      item_category2: 'T-shirt',        // if it is possible to drill down the categories (e.g. Apparel, then T-shirt, then Men), use item_category2, item_category3, etc. Can use from item_category up to item_category5
      item_variant: 'Blue',             // insert an actual product variant
      quantity: 1                       // product quantity in the cart
    },{
      item_name: 'Product 2',
      item_id: 'product2',
      price: '12.99',
      affiliation: "Merchandise Store",
      item_brand: 'Brand B',
      item_category: 'Category B',
      item_variant: 'Yellow',
      quantity: 1
    }]
  }
});