Using Basic Auth with HTTP Query Smart Service

I have requirement to invoke the external rest webservice using basic auth with HTTP Query Smart Service. I need to read the credentials from a third party secure store. I have a Java Plugin to do so, however I see in the HttpAuthenticationBasic is Custom Data Type wth FieldKey, externalSystemKey. Any Idea can I use expression for the username/password to pull it from java plugin?. I see "Send HTTP Request" works for me and the Credentials are passed as simple text. I need to achieve the Http Basic Authentication using the "HTTP Query Smart Service". I tried giving the username and password value in the SCS username and SCS Password and I am getting below exception.

<a title="Title: Failed to retrieve value from SCS username=, Message: ObjectNotFoundException[]: The External System [identifier=] does not exist, has been deleted, or you do not have sufficient privileges to access its data. (APNX-1-...

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