Add Item(s) to Cart with Custom Fields using the commerce/cartApi in Salesforce Core Commerce

addItemsWithCustomFieldToCart.js

// Using add itemS to cart from the commerce/cartApi
import { addItemsToCart } from "commerce/cartApi";

// Set up an array as a payload.
const payload = [{
  productId: this.productId,
  quantity: this.quantity,
  type: "Product"
}];

// Add custom field data to the CartItem record.
if (this.customMetric) {
  // Only sending one product in this example.
  payload[0].customFields = [
    {
      attributes: {
        type: "CartItem"
      },
      CustomField__c: this.customMetric
    }
  ];
}

const result = await addItemsToCart(payload);