Text filtering issue..ASAP

For that you need to handle the keyboard events, something like this.

control.KeyPress += (sender, e) => 
{
   // Get the key, if it is Y
   // then Filter the result. 
}

Sadly, we cannot share an entire application project under the Quick answers tab, that is why there are articles on C# Corner. So, I would like to ask you to search for

Handling keyboard events in C# (add the framework being used, WPF or WinForms etc.)
Filter the ComboBoxItems based on a value.

These would help you in finding a good solution. The KeyPress event can be learnt about from MSDN itself.