In the current implementation of the control you cannot apply the append functionality with TextSearchMode set to "Contains". If we add a text before the typed symbol it will affect the Search functionality of the control and in many cases the result will be not relevant.
There is another of our controls which can be suitable for your purposes - RadAutoCompleteBox. The autocompletion in "Contains" mode will still not work as you are expecting but there is a dropdown where the filtering can be seen. In the link pasted below you can find an example showing how to equip the control with a dropdown button so it will be more like RadComboBox. Please, take a look at it -
We hope that the proposed solution will be suitable for your project.
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