<?php
$this->EE =& get_instance();

$this->EE->load->add_package_path(PATH_THIRD.'cartthrob/');

$this->EE->load->library('cartthrob_loader');

$params = array(
	'product_id' => 37, //your entry_id
	'item_options' => array(), //ie, 'product_size' => 'L',
	'quantity' => 1,
	'class' => 'product',
);

$this->EE->cartthrob->cart->add_item($params);