To add customer, order, variant, and product metafields, follow these steps:
1. Click "Add Field" and search for the metafields you want to add.
2. Add the necessary "metafields" fields.
3. Enter the namespace and key in the provided field.
For example: "namespace.key" (as in the sample of the image below)
How to obtain the namespace and key?
Obtain the namespace and key from Shopify admin under Settings > Custom Data > Metafield, then define the metafield in the specific store section.
If you would like to add the metafield in xml format, you can use this method in syntax <artikel_id>{{ item['Product Variant Metafields (User-defined) '] }}</artikel_id>