how can I add js function in button body?

-- HikaShop version -- : 2.6.3
-- Joomla version -- : 3.4.8
-- PHP version -- : 5.5

Hello,

$ajax = 'onclick="var field=document.getElementById(\'hikashop_product_quantity_field_'.$i.'\');'.$ajax.'" ';

In this string i must to add sum js code for google analytics

This code onclick="ga('send','event','zakaz', 'zakazal');"
When i add this code my button is not work(cannot add goods in basket).
This is need for Google Analytics goal to add to the press of a button.
This is need for to track conversions
how can I add js function or this string (onclick="ga('send','event','zakaz', 'zakazal');" ) in button body?
Regards

Thank you, this is what I need