custom_my_form($form, &$form_state) {
        $form['user_firstname'] = array(
            '#type' => 'textfield',
            '#title' => 'ID',
        );
        $form['user_email'] = array(
            '#type' => 'textfield',
            '#title' => 'Email',
        );
        $form['user_pwd'] = array(
            '#type' => 'password_confirm',
            '#required' => TRUE,
        );
        $form['submit_button'] = array(
            '#type' => 'submit',
            '#value' => 'Submit',
        );
      return $form;
    }

    function custom_my_form_submit($form, &$form_state) {
       //Here it is working fine,
        $fname = md5($form_state['values']['user_firstname']);
        $email = md5($form_state['values']['user_email']);
        $pwd = md5($form_state['values']['user_pwd']);
        $values = array(
            'Form_Req' => array(
                'Header' => array(
                    'Info' => 'xyz',
                ),
                'Body' => array(
                    'Type' => 'FromSubmit',
                    'firstName' => $fname,
                    'lastName' => $email,
                    'emailId' => $pwd
                )
            )
        );
        $json = json_encode($values, JSON_UNESCAPED_SLASHES);
        $options = array(
            'headers' => array(
              'Content-Type:application/json',
             ),
             'method' => 'POST',
             'data'=> $json,
         );
         drupal_http_request('http://example.com', $options);
    }