PostJSON with user authentication

I am looking for a sample code invoking PostJSON service with user authentication across session.

i.e Once user is authenticated during first use(login) subsequent REST calls should use the current user(session user) &password.

How do i pass the username & password to PostJSON, dynamically by taking it from the current session

var params = {

proxyScheme: undefined /* STRING */,

headers: undefined /* JSON */,

ignoreSSLErrors: undefined /* BOOLEAN */,

useNTLM: undefined /* BOOLEAN */,

workstation: undefined /* STRING */,

useProxy: undefined /* BOOLEAN */,

withCookies: undefined /* BOOLEAN */,

proxyHost: undefined /* STRING */,

url: undefined /* STRING */,

content: undefined /* JSON */,

timeout: undefined /* NUMBER */,

proxyPort: undefined /* INTEGER */,

password: sessionpassword?? /* STRING */,

domain: undefined /* STRING */,

username: sessionuser?? /* STRING */

};