var form = new WWWForm();
form.AddField( "name", "value" );
var headers = form.headers;
var rawData = form.data;
var url = "www.myurl.com";
// Add a custom header to the request.
// In this case a basic authentication to access a password protected resource.
headers["Authorization"]="Basic " + System.Convert.ToBase64String(
    System.Text.Encoding.ASCII.GetBytes("username:password"));
// Post a request to an URL with our custom headers
var www = new WWW(url, rawData, headers);
yield www;
//.. process results from WWW request here...

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
    public WWWForm form = new WWWForm();
    public System.Collections.Hashtable headers = form.headers;
    public byte[] rawData = form.data;
    public string url = "www.myurl.com";
    public WWW www = new WWW(url, rawData, headers);
    IEnumerator Example() {
        form.AddField("name", "value");
        headers["Authorization"] = "Basic " + System.Convert.ToBase64String(System.Text.Encoding.ASCII.GetBytes("username:password"));
        yield return www;
    }
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

    public form as WWWForm = WWWForm()

    public headers as System.Collections.Hashtable = form.headers

    public rawData as (byte) = form.data

    public url as string = 'www.myurl.com'

    public www as WWW = WWW(url, rawData, headers)

    def Example() as IEnumerator:
        form.AddField('name', 'value')
        headers['Authorization'] = ('Basic ' + System.Convert.ToBase64String(System.Text.Encoding.ASCII.GetBytes('username:password')))
        yield www