After that you can add this extension field to given UI.
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.
Choose Business Context "Product Master General".
Wait until the status has changed to "Published":
The publish is done in an asynchronous way:
Create a new extension field in this tile:
The publish needs to be done by button "Publish":