You need to use the GET method to retrieve data from a web page. Why do you need to use POST to get data?
Have a look at the REST API app , it can get (that is fetch) any data from a web site exposed as an api
I'll add that the Website Input app does allow you to submit form elements for logins, if that is why you wanted to perform a POST.
As @Sukisen1981 noted, the app doesn't currently support POSTs otherwise.