How to create drop down list for a property in OPPID CONNECT EDITION

I was trying to create a drop down list for a property I added to PID schema, but I can't find value map in Classes as in OPM

Then, I tried to using the StandardValues Attribute to accomplish it. However, it seems sth was missing under this attribute. As I remembered, there should be a value map under this attribute, and able to be add element.