Problem: How to create a calculated custom field in a standard app.

In the standard app F3893 Manage Sales Orders - Version 2, at item level, in the “Advanced Data” section, I need to add a custom field “Tax code”, which must be calculated based on the data available in “Price Elements”. These data can be modified dynamically; in particular, if the price condition MWST is present, I read the corresponding value of the “Tax code”. (Image1) To do this, I created the Data Source Extension.