functions.php

<?php

function write_csv_register_rest_route()
{
	register_rest_route('write_csv/v1', '/process', array(
		'methods' => 'POST',
		'callback' => 'write_csv_process_callback',
		'permission_callback' => 'validate_request_origin'
	));
}

function write_csv_process_callback($request)
{
	$parameters = $request->get_json_params();

	$data = "";
	$first = true;
	foreach ($parameters['data'] as $value) {
		if ($first) $data .= "{$value}"; 
		else $data .= ",{$value}"; 
		$first = false;
	}
	$data.="\n";

	file_put_contents(__DIR__ . '/assets/'.$parameters['fileName'],
	$data
	,FILE_APPEND);
	
	return new WP_REST_Response('success', 200);
}

add_action('rest_api_init', 'write_csv_register_rest_route');



// validate_request_origin: 
// https://gist.github.com/artemsites/30a03ed3047e63ccb0db20fd781474e4