Hi mh Chen,
New attributes or calculated attributes only exist on the scope of aggregates.
To add an aggregate to your screen, you can right click the screen where you want to have the dropdown and choose the option Fetch from the database:

After that, you can drag your entity PullDown to the aggregate, and now you'll have the option New attribute. After you select the option New Attribute, you can set its value and name properties on the right bottom panel like on the image below:

After that, you will be able to set the List of the dropdown as the query and have the calculated attribute (in my example DisplayName) as an option, as shown on my previous message.
Kind Regards,
João