Example swift-nio-ssl using passphraseCallback?

Trying to setup TLS server and need to provide passphrase for private key.
I'm having a mental block and can't seem to figure out how to use the passPhrase callback. Any examples available ?

You'd use it like this:

let context = NIOSSLContext(configuration: configuration) { setter in 
    setter("my passphrase".utf8)
}

thanks that got me moving forward again.