it’s the same *.conf file you would use for a standard mqtt bridge.
Mine (for cloudmqtt) looks like this (just fill out the stuff in “<>”):
connection cloudmqtt
address <server_address>:<server_port>
remote_username <username>
remote_password <password>
clientid <client_name>
try_private false
start_type automatic
topic # in
check out this topic for further details.