custom javascript in billing/shipping address

-- HikaShop version -- : 2.6.2
-- Joomla version -- : 3.4.8

Hi ,
I would like to implement special behavior in the address fields,
I want to auto-fill the state and country depending on the zip code, I have worked the javascript code out, and tested it in a simple html page, but I can't get it to run inside hikashop checkout .
what is the exact view file to update? either I can't find it , or it's being added but not running properly,
should I add the script tag ? or use the php addScriptDeclaration function, to insert javascript as string?

Thanks, it worked fine indeed