Key fields
In the external data source editing window, in the Key fields field, click the selection button.
To add a key field, move it to the Selected fields list by clicking the > (Add field...) button or double-clicking the field name in the Available fields list.
To remove a key field, move it to the Available fields list by clicking the < (Remove field...) button or double-clicking the field name in the Selected fields list.
If you add multiple key fields, you can change their order by clicking the arrow buttons above the Selected fields list.