Here is the wget command :
wget --auth-no-challenge --http-user=auth_user --http-password=auth_pass --save-cookies cookies.txt --keep-session-cookies --post-data 'user_login=username&pass=password' https://www.helloworld.com/login.php

Here is the groovy script:

static List<String> login(String baseUrl, String path, query, method = Method.POST) {
        try {
            def ret = null
            def http = new HTTPBuilder(baseUrl)

            http.request(method, ContentType.URLENC) {
            uri.path = path
            uri.query = query

            headers.'Authorization' = "Basic ${"auth_user:auth_pass".bytes.encodeBase64().toString()}"

            // response handler for a success response code
            response.success = { resp, data ->

                println "response status: ${resp.statusLine}"

                resp.getHeaders('Set-Cookie').each {

                    def cookie = it.value.split(";").toString()
                    println cookie
                    .add(cookie)
                }
                println 'Response : ' + resp.getData()
            }
         }
         return cookies

        } catch (groovyx.net.http.HttpResponseException ex) {
            ex.printStackTrace()

        } catch (java.net.ConnectException ex) {
            ex.printStackTrace()

        }
    }

def url = "https://www.helloworld.com/login.php"
def path = ""
def query = [ user_login: "username@calypso", pass: "password" ]

// Submit a request via POST
def response = login(url, path, query)