--insecure - Allows insecure SSL connections and transfers to be performed. In case your proxy doesn't have a CA certificate or has an invalid one you need to use this flag.
<Url> is the full url of the proxy. For example, http://127.0.0.1:8888. If you do not provide the url when running the command, the NativeScript CLI will prompt you to provide it.
<Username> and <Password> are your credentials for the proxy. These are not necessary, however, if you provide a <Username> you need to provide a <Password> too.