Send email with Mailgun with Javascript

I'm working with JS and I have the follow code:

  var XHRResponse = XHR.send("POST", "https://api.mailgun.net/v2/sandbox224f28ae45a8499d84184fd4c48e62ee.mailgun.org/messages",{"parameters": {"from": "Mailgun Sandbox <postmaster@sandbox224f28ae45a8499d84184fd4c48e62ee.mailgun.org>","to": "Jordi <myname@gmail.com>","subject": "Hello Jordi","text": "Congratulations Jordi, you just sent an email with Mailgun!  You are truly awesome!  You can see a record of this email in your logs: https://mailgun.com/cp/log .  You can send up to 300 emails/day from this sandbox server.  Next, you should add your own domain so you can send 10,000 emails/month for free."    }  }); 

I need to authenticate with the apikey but i don't know where and how.

Thanks