PAC/JS File with Basic Authentication (JS Coding help)

Question

I am trying to send username:password to a proxy server. My JS file is as such:

function FindProxyForURL(url, host) {
  if (dnsDomainIs(host, ".mydomain.org"))
     return "DIRECT";

 return "PROXY username:password@serverip:3128";
}

I have tested username:password@serverip:3128 with a proxy checking tool and it connected successfully using the plaintext username and password. I also checked the connection logs on the proxy and it sees the connection, so I know it's not the credentials or the server. It's got to be this code.

Any help is greatly appreciated. Thank you!