How to get export value of a listbox / combobox

Hello!

Can anyone tell me, how I could get the ExportValue, chosen by the user, of a listbox rsp. combobox?

I have a list ob Values in the listbox, for example

Jan, Feb, Mar, ...
And I need to get the defined export-value,
01, 02, 03, ...

Neither "this.event.value" nor getField("fieldname").value returns the export-value "03", but the value "Mar".
I did not find any description for this problem in the js-documentation...

Thank you for help!

Dieter