How to have one text field, the text could be typed in or select

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I would like create a text input field in a <form>, in asp, only one input field, the user could type in the data, or user could select the data from a dropdown box?

I don't like to use a Combo Box, because I like to have only one input field on the screen.

Before I thought this is a common required in asp form design. I did this topic search for two days. I believed I miss something here. Could anybody help me to figure out how to do it?

Thank you.