How to send a POST request using HTTPie?

Just to clarify what kOpernikus said, when you are making a POST request using httpie, use the following syntax:

   http --form post :3000/register username="gilbert" password="stackoverflow!"

Alternatively, since forms are for post requests you can leave out post and also abbreviate --form to -f like so:

   http -f :3000/register username=gilbert password=stackoverflow!

EDIT (thanks to Aerials)

To pass csrf token as header in the post request do:

http --form POST http://localhost:8000/login/ username=user password=pass X-CSRFToken:assQ$%auxASDLSIAJSd

It was an httpie usage problem as the form flag was necessary, as silex requires the parameters to be form-encoded, yet the default of HTTPie is to pass a JSON object.

$ http --form POST http://localhost:1337 data="hello world"

HTTP/1.1 200 OK
Cache-Control: no-cache
Connection: close
Content-Type: application/json
Date: Wed, 14 Oct 2015 15:04:09 GMT
Host: localhost:1337
X-Powered-By: PHP/5.5.9-1ubuntu4.13

{
    "message": "hello world"
}