Thanks @danielbartel - you can add it in the settings of Admin > custom JS. Just paste it in there and it will appear on every page (as opposed to pasting it in the META drawer of the page which will just add it to that single page)
But you may as well add it to the template files.
Add the JS file to the Template folder then link to it in the layout.php with a reference like:
`<script src="<?php echo $path; ?>/template/js/my-js.js"></script>`
Hope that helps!