select widget with OptionList process variable

Hello,

I want to display a OptionList text variable in a Select widget. It seems to be quite simple but I have a couple of questions. I found an example here but with a JSON object instead of an Option list variable.

Next things I did in Studio 7.1.5 community.

1) Create a Option list variable (process variable) with a list of text options. called: "cars". ie.

Alfa
Renault
Mercedez

2) In a Form UI 7.1.5, I create a link to a process variable called "CarList:

../API/bpm/activityVariable/{{taskId}}/cars

3) In a select widget I configured:

3.1 Available values = CarList

Questions:

Which values should I put in "Display Key" and "Returned key" ?

Thanks in advance