How to specify key in CDS view?

I defined a CDS view based on an entity. When I activate, it says:

Missing specification of keys for view "sap.ms.data::DB.ApplicationView".

However, I am at a lost how the specify key in CDS View. The developer guide did not mention anything. If I put a key before one of the attribute:

Cannot find sub artifact "key" in "Application"

My CDS view is as follows:

view ApplicationView as select from Application {

  ApplicationId,

  AccountRef.AccountId,

  DisplayName,

  Version

};