Add custom Javascript / HTML

Hi Geoffrey,

Not sure I fully understand your question here, but I assume Block element or Hook element is what you are looking for.

Block element: https://docs.generatepress.com/article/block-element-overview/
Hooks element: https://docs.generatepress.com/article/hooks-element-overview/

And here's a visual hook guide to help you choose the right hook:
https://docs.generatepress.com/article/hooks-visual-guide/

Set location as Post > all post in display rule.

Let us know if you need further assistance.