Send data with cURL by post's data

Found out. I used json encode just like @ADyson suggested. Here is the code if anyone need it:

$data = [
    'contact' => [
        'dataFields' => [
            [
                'key' => 'FIRSTNAME',
                'value' => $_POST['firstName']    
            ],
            [
                'key' => 'LASTNAME',
                'value' => $_POST['lastName']  
            ],
            [
                'key' => 'FULLNAME',
                'value' => $_POST['lastName']  
            ]
        ],
        'email' => $_POST['email'],
        'optInType' => 'Single',
        'emailType' => 'PlainText',
    ]
];

so you can use like this:

CURLOPT_POSTFIELDS => json_encode($data)