Add (All Customers) to DropdownList

I'm not sure how to do this in Radzen UI.

I have a list of Customers from the database - for ease, CustomerCode & CustomerName and I display this in a dropdownlist. This is fine. But I want to add to this list the first item to be "All Customers".

The only way is to add additional item to your database list:
(new List(){new YourItem(){}}).Concat(yourDatabaseList)