REST API - client id

Hi. I'm trying to migrate some code from the old soap api to the new rest api. I have a chunk of code that does this:

create client
create site
create ftp
create db user
create db

this all works fine up until create db user, I am trying to send the client ID to associate with it, but the db user account gets created in ispconfig with no client. Heres my old code that worked properly:

PHP:

$site_result = $this->api_request("sites_database_user_add",
            array(
                    $this->session_id,
                    $clientId,
                    array(
                            'server_id' => $serverId,
                            'database_user' => $dbUser,
                            'database_password' => $dbPass

                        )
                )
            );

And my new code (using guzzle in laravel)

PHP:

  $dbUser = $gclient->request('POST', $ispUrl.'sites_database_user_add', [
        'json' => [
          'session_id' => $session_id,
          'client_id' => $clientId,
          'params' => [
              'server_id' => $server_id,
              'database_user' => $dbUsername,
              'database_password' => $dbPassword
          ]
        ]
      ]);

I've tried a few things putting client_id in params etc but it seems like I'm not sending the client_id in the correct way, has anyone got any ideas to solve this? Thanks