Item Cross Reference Type - How to prevent users from entering any Values in Value Set

Hi Gurus,

I can define my Cross Reference Type without any problem. However, users had the power to enter any values in the Value field.

From now on, we would like to restrict the users to select only pre-defined values from this List of Values.

Let me know what I need to change on this Key Flexfield and where this setup is made.

Thanks a lot

Cheers

Philip