You can add it to header.tpl or footer.tpl depends if you want it before closing head or body tags.
in case your script has curly braces "{ ... }" like the async one you posted, than you should wrap it inside literal tag. e.g.: {literal} your script {/literal}
Good luck.
p.s. I believe that @SLiCK_303 suggestion would work as well, but I never tested that feature so I can't tell.