Finally I found the answer of this so for the sake of other user so they may find a help from this post in future I'm submitting the answer.
the url
is this one.
https://api.mailgun.net/v2/yourdoamin.com/messages
username = api
password = {your key}
Now after that set your content type:application/x-www-form-urlencoded
.
Now post the data. for formatting the data you can use this link Click Here
Demo data format
from%3D%27Excited%20User%20%3Cme%40yourdoamin.com%3E%27%20%5C%0A%20%20%20%20%20to%3Dbaz%40example.com%5C%0A%20%20%20%20%20to%3Dbar%40example.com%20%5C%0A%20%20%20%20subject%3D%27Hello%27%20%5C%0A%20%20%20%20text%3D%27Testing%20some%20Mailgun%20awesomness!%27