"JSONString" => "{"customer_name":"9a52ef41-df2b-42e9-ad8f-383f03208878","billing_address":{"address":"Foo 21","street2":"","city":"Palma","state":"Palma","zip":"77010","country":"Spain","phone":"099 999 999 999"},"contact_persons":{"email":"john@example.com","phone":"099 999 999 999"}}"

And this is the code calling the API.

public function request($url, $method = 'GET', $data = null)
{
    $options = [
        'query' => [
            'authtoken' => $this->token,
            'organization_id' => $this->organization,
        ],
    ];

    if (! is_null($data)) {
        $options['headers'] = [
            'Content-Type' => 'application/x-www-form-urlencoded;charset=UTF-8',
        ];

        $options['form_params'] = [
            'JSONString' => json_encode($data),
        ];
    }
    
    try {
        $request = $this->client->request($method, $this->api_url.$url, $options);

        return json_decode($request->getBody()->getContents(), true);
    } catch (Exception $exception) {
        throw $exception;
    }
}