endpointSecretKey

The endpoint's own identity key (32 bytes) — not your API secret. The access token is scoped to it, so pass the same key you persist for your endpoint's identity. A fresh key is generated when omitted.

Set the key here, not via EndpointOptions::secret_key or EndpointBuilder::secret_key: both are layered on top of the preset and would replace the one the token is scoped to. Doing that is an error, not a silent auth failure — this preset pins the key.