Accessing private key within HSM

[Kitty-Hawk1]

Attempting use a private key stored within HSM via openssl engine, how should the private key reference be passed in ?

Example in openssl

openssl dgst -sign 0xe0f1 --keyform engine -engine trustm_engine -out helloworld.sig helloworld.txt

  bridge_cafile /etc/mosquitto/ca_certificates/RootCA.crt
  bridge_certfile /nvmem/CONN_uP.crt
  bridge_insecure true
  bridge_keyfile 0xe0f1
  bridge_protocol_version mqttv50
  bridge_tls_version tlsv1.3
  notification_topic CloudState
  notifications true
  restart_timeout 1
  start_type automatic
  tls_engine trustm_engine
  tls_keyform engine

Mar 24 17:25:36 mach-cw-rnet-sn-gpm-249-2050 mosquitto[235]: 1648142736: Error: Unable to load client key file, check bridge_keyfile "0xe0f1".

	ret = SSL_CTX_use_PrivateKey_file(mosq->ssl_ctx, mosq->tls_keyfile, SSL_FILETYPE_PEM);
	if(ret != 1){
#ifdef WITH_BROKER
			log__printf(mosq, MOSQ_LOG_ERR, "Error: Unable to load client key file, check bridge_keyfile \"%s\".", mosq->tls_keyfile);