Hey there, this is later done through the reference selector within your page, here you select an object over association,
what you will have to do later on is create an entity that has a one to many association to the entity in which you store the customer name.
Parent Entity | 1 → * | Customer entity
You will then create a page in which you have a data view for the parent entity and a reference selector for the customer entity (where the displayed attribute is the name). This will give you the wished dropdown :)
if this answer is sufficient, please mark it as the answer and if you have any questions, do ask!