var username = someString.substr(0, index);
var password = someString.substr(index + 1);

body: JSON.stringify({
           "ctype": "Username",
           "cvalue": username,
           "password": password
       }),

body: JSON.stringify({
           "ctype": "Username",
           "cvalue": "Usernamehere",
           "password": "passwordhere"
       }),