How can I submit form using REST API and what should I pass in $data in REST API

<?php

$eloquaRequest = new EloquaRequest('site', 'user', 'password', 'baseUrl'); 

$data = ...

$response = $eloquaRequest->post('/assets/email', $data);

?>

what value should I pass in $data and how, suppose I have 4 field in my form, firstName,lastname,emailAdress and Business phone, So how can I pass these value to API