using a textbox to select a value from a dropdownlist

 Dear all;
I have searched everywhere on how to go about this but I havent been successful
I have a textbox and a dropdownlist
my dropdownlist contains three items which are red, blue and orange
Now, i would like a situation whenever the user type in blue in the textbox, it should assign the dropdownlist.selecteditem.value as blue.
How do I go about doing that. Thank you.
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