I think the nearest thing to what you want could be to use the combo box as a “Dropdown list” (instead of Dropdown combo).
But this way the user can only select elements already in the list.
This property is called “Style” and is part of the Appearance cathegory.
But there is another option (which I think will do what you want), and that is to use another control.
There is one called “Solar Winds Auto Completion ComboBox”, or AutoCombo.
I currently have this component on my machine, but to be honest, I don’t remember how I got it. Maybe a google search could help to get it.