How to edit Duplicate drop down values

Hi Allan.

I want to edit the dropdown options after pressing duplicate..

just like here:
https://editor.datatables.net/examples/api/duplicateButton.html

i want to show the key next to the title.
for example:
now it showing like this:
Edinburgh
London

and i want to show like this:

1 - Edinburgh
2 - London

How can i do that?

Thanks