I have been advised to place the following code in my site,

<script async="true" src="https://www.whisk.co.uk/app/assets/whiskbutton.js" type="text/javascript"></script>

but using the Editor function within WP where on earth would I place the code? They suggest appending it to something called <div id=“footer”> but I fail to find such a div container in footer.php and I have no idea what they actually mean by appending the code to it, as in the where, before, after, within?

I'd suggest putting it in the ATO->Add HTML/CSS Inserts->HTML Inserts: Body Bottom option