Hello @Mike_de_Block;
As @samliew points out in How to update meta title and meta description on shopify blog - #5 by samliew, you’ve got to look at your Shopify configuration to find the Metafield ID that you want to work with.
Shopify has a list of List of standard definitions and the metafield ID is on the left. Ex: 2 for ISBN.
In case you are dealing with custom metafields, there’s good documentation, too.
