Hi Richard,
Thank you for your update.
There is no any separate property to define primary key in the AutoComplete Textbox.
But the functionality of the Primary key can be achieved. The below sample link helps you to do the same.
http://websamples.syncfusion.com/samples/Tools.Web/5.1.0.51/Forum-66608/main.htmIn the sample 'AutoCompleteTextBox' is populated with the values of the XML file. The sample check for the unique name for the field 'FirstName'. The List binded to the AutoCompleteTextBox is checked for the repeated values, based on the result the item gets added to the AutoCompleteTextBox.
Please let us know if this helps you.
Regards,
Valli