in the text change event of text box call auto fill suggestion of combo box
eg auto complete..<a href="http://www.c-sharpcorner.com/blogs/2050/autocomplete-combobox-in-visual-c-sharp-2010.aspx">http://www.c-sharpcorner.com/blogs/2050/autocomplete-combobox-in-visual-c-sharp-2010.aspx</a>