How do I add reading time to massively theme?

I tried to follow this guide Tutorial: Show reading time & progress in posts I inserted all the code in Code injection. I got no results.
Massively demo site:
I am trying to get the reading time to show up on this theme

I’d customize the theme files directly and upload this, e.g., add something similar to <span class="reading-time">{{reading_time minute=(t "1 min read") minutes=(t "% min read")}}</span> to post.hbs. You’ll need to decide on the style and positioning, and add a <style> … </style> block to the footer under code injection.

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would I do this in code injection in the ghost editor?

No. Follow the instructions you posted, and download and edit the theme files.