How to pop up a textedit box after selecing a dropdown item

[Guest]

Hi,

In my dropdown, there is one selection which if chosen will require the user
to type in a new value that I'dd like to ultimately add to the dropdown.

How do I pop up a little dialog box with a text field to type data in.

Thanks for any suggestions...

..

 

[Guest]

I am using c# with ASP.NET