How to use Attributes with "Selector" Control Type ?

Dear ALL,

For example, I have the following situation:

Step 1: I created a  Attributes as below:

• AttributeID: FACODE
• Description: Fixed Asset Code
• Control Type: Selector
• Schema Object: PX.Objects.FA.FixedAsset
• Schema Field: AssetCD

Step 2: I created Case Class with name INTERNAL using “FACODE” AttributeID 

Step 3: I created Case using “INTERNAL” Class ID. In the ATTRIBUTES TAB > Adding Attribute “Fixed Asset Code” > In the Value => I didn't see the selector ?

Please help me How to use Attributes with "Selector" Control Type ?

Note: Acumatica Verion: 2020 R1

Regards,

NNT