Curl login to web page

Hello dears,
I am trying to log in the website using curl but no luck so far.
The web page Content-Type is : text/html;charset=ISO-8859-1

I try the following command using curl:

curl \
--header "Content-type: text/html" \
--request POST \
--data '{"user": "someusername", "password":somepassword}' http://someweb.com/login.htm 

What happens is that it just dumps the web page source code. It even does not try to authenticate.

Thanks in advance.