create iOS 12 NWConnection that uses client cert

I'm trying to set up a client connection to communicate with an MQTT broker using client side certificates. I've been able to proof-of-concept this all on the Linux side using command line. The MQTT broker is set to require client cert, and a command like:

mosquitto_pub -h servername -p 8899 -t 1234/2/Q/8 -m myMessage --cafile myChain.crt --cert client.crt --key client.pem

does the job nicely. Now I'm trying to set up an NWConnection:

self.connection = NWConnection(
    host: NWEndpoint.Host("servername"),
    port: NWEndpoint.Port(integerLiteral: 8899),
    using: .tls)

But I think that simple .tls class var needs to be a much more involved NWParameters object, but I'm at a complete loss (documentation is pretty sparse) as to what I create there to attach the client certs to the parameters. Nor do I know how I even move from .crt/.pem file to something the app manages programatically.

What is an example of how one would configure the NWParameters to support the client certs?

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