How To I Add Button Control In Combo Box Using C#

This is not a good question - we cannot work out from that little what you are trying to do.
Remember that we can't see your screen, access your HDD, or read your mind.
So what exactly do you want? A button to do what? If it's to be part of the selection (i.e. you can drop down and press the button instead of selecting it) then that's ... um ... somewhat esoteric ... and not necessarily obvious to the user.
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