problem with Simple style combobox
 

Hi all,
please help me about Simple type of ComboBox.
<pre>
Simple Stype comboBox Layout.
----------------|
Monitor \/ <- type monitor and hit enter
----------------|
|Computer |
| Proccessor |
| Monitor |
-----------------

</pre>
This is a SIMPLE Type of ComboBox.
When user type Listed item "Monitors" and hit enter, in that case Listed monitors must be highlighted.

i did alot but nothing happend.
in default "user type listed item and press down key its work" but i don't want that i need to effect by Enter key.
Please help me.
Sincerly Bir