port does not need to be explicitly assigned.
pr_run(pr, host = "127.0.0.1", port = getOption("plumber.port", NULL))
A Plumber API. Note: The supplied Plumber API object will also be updated in place as well as returned by the function.
A string that is a valid IPv4 or IPv6 address that is owned by this server, which the application will listen on. "0.0.0.0" represents all IPv4 addresses and "::/0" represents all IPv6 addresses.
A number or integer that indicates the server port that should be listened on. Note that on most Unix-like systems including Linux and Mac OS X, port numbers smaller than 1025 require root privileges.