Bit more digging and the following steps work.
Install mkcert and generate certificate
https://github.com/FiloSottile/mkcert
Configure server
replace run() in the post above with
function run()
tlsconfig = MbedTLS.SSLConfig("cert.pem", "cert_key.pem")
HTTP.serve(requestHandler, "0.0.0.0", parse(Int, APP_PORT), sslconfig=tlsconfig)
end
The https endpoint can then be accessed.