How to add HTML snippet

I would like to add Zendesk support widget on my web app. Here are the instructions:

" To make Web Widget visible to customers, update the HTML of every page you want it on. Just paste this code snippet in before the closing </body> tag"

Does someone know where I can find the ‘body’ tag please? Or is there a plugin I can use to enable me to do this?

Thank you in advance!

Just drop HTML element somewhere on the page and paste your code