The value that you should provide on the agent side is the same un-encrypted value that you provided when creating the secret_token value on the server.
What this secret token provides is a way to make sure that no other agents that the ones you have set with this secret token can send APM data to your server.
While this value is stored in plain-text on the client, only the agents that are under your control would have it and will be able to send data.
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