To add the metafield, please follow the steps below:

1. Click on the Add Field button and search for the metafields

2. Then, try adding the "metafields" field. 

3. It will navigate to this page and you need to enter the namespace and key

How to get the namespace and key?

Shopify admin Settings > Custom Data > Metafield 

Click on the specific section of your store to add the metafield definition. Once created, you can use the namespace and key to define the field.

For example: "namespace.key" (as in the sample of the image below)