When "eating your own dogfood" like the Soundcloud API how does the client get the private key?

When "eating your own dogfood" like the Soundcloud API how does the client
get the private key?

I noticed that Soundcloud consumes it's own API. Since we are not supposed
to transfer the private key over the wire, how does it make sure the
client has private key to encrypt messages to send. Regarding the
situation where one is consuming one's own API?