Adding Address Lookup to the checkout page

I want to install an address lookup (by postcode) to the checkout to speed up the checkout process,

i have an account with a company that provides me access to there Address DB via JavaScript, it really simple to install normally,

i just need to place the .Js file in the head of the page an specify a few variables (postcode input id ect…)

however im not sure which page i need to edit to include the .Js file in the checkout page? if anyone could help it would be much appreciated.