Need to installed a script for a pop up to work in upfront theme

Hi, Good day to you. I need to installed a script at the homepage.

<script src="//a84ba911b74c4830863e8b8d971d153b.js.ubembed.com" async></script>

Where can I paste it? Because according to the instruction as seen below:
http://documentation.unbounce.com/hc/en-us/articles/214545243#setup-script

I need to paste it at the top of the post. But I understand that upfront generate the homepage. So, where can i paste the script?
Many thanks.