The address (host[:port]) of the IRC server. senpai uses TLS connections by default unless you specify tls option to be false. TLS connections default to port 6697, plain-text use port 6667.
URircs://, irc://, and irc+insecure:// URLs are supported, in which case only the hostname and port parts will be used. If the scheme is ircs/irc+insecure, tls will be overriden and set to true/false accordingly.