Hi Helmut,
We have reviewed your query, and currently, there is no direct support to select an item that is not in the ItemSource. However, you can manually enter an item and press the Enter key to add the item to the ItemSource in the KeyUp event. A sample has been attached for your reference. If you require any additional assistance, please feel free to reach out.
Regards,
Madhumithan Jayapathi
Attachment:
ComboBoxAdvWpf_d3e8a4b2.zip