Create a new extension field in this tile:

Choose Business Context "Product Master General".

Just choose Text as extension field type and length: 20. Once created, it has status "Not Published". In order to allow UI to add this field, you have to publish it.

The publish needs to be done by button "Publish":

The publish is done in an asynchronous way:

Wait until the status has changed to "Published":

After that you can add this extension field to given UI.