Thank you for writing.
We have an Knowledge Base article for this. There you can find a detailed explanation and example for such scenario. You can see the article at the following page: Use custom filtering to search in all columns of RadMultiColumnComboBox.
Please note that when you press enter to select a row the specified by the DisplayMember
property field will be shown in the textbox.
If you continue to experience any issues with this please open a support ticket and attach your project to it. This will allow us to investigate your exact case and provide you with a solution for it.
I hope this helps.
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