in your nginx config

location /urls {
    proxy_pass 127.0.0.1:4200;
}

in your weechat config

  plugins.var.python.urlserver.http_hostname            string  "0.0.0.0"
  plugins.var.python.urlserver.http_hostname_display    string  "kame.us"
  plugins.var.python.urlserver.http_port                string  "4200"
  plugins.var.python.urlserver.http_port_display        string  "80"
  plugins.var.python.urlserver.http_scheme_display      string  "http"
  plugins.var.python.urlserver.http_url_prefix          string  "urls"