First it may be that the username/password are wrong and don't match those in the blockchain's multichain.conf file – I'm not sure if the values you placed above are a placeholder or that's what you really entered.
Second if you use the help getinfo command in multichain-cli you will see an example of curl usage.
Lastly if you run curl with the -vvvv option you'll see much more details about the process and what's going wrong.