SQL Server Always Encrypted - Create an external table containing an encrypted column
First:


  Before creating a database scoped credential, the database must have a
  master key to protect the credential. For more information, see CREATE
  MASTER KEY (Transact-SQL).


CREATE DATABASE SCOPED CREDENTIAL

So the Database Master Key is used to encrypt the credential.

Second:


  I'd like the [AlwaysEncrypted] column to be unencrypted when querying MyTable from Elastic Database Query.


That is not possible.  AlwaysEncrypted is intended to have the data decrypted only on the client.  Even in the case where SQL Server is the client, there's no way to store the encryption key inside SQL Server and use it in the database engine.  There's no fundamental reason why this couldn't be implemented.  But the main use case for AlwaysEncrypted is storing data in such a way that SQL Server (and its administrators) cannot access this data.