Add html code to all the posts after specific paragraph

Hello,

I'm trying to add an email sign-up form to all my posts and I don't see a way to add it to all the posts at the same time.

I would like to add this email sign-up form to all the posts after a specific paragraph or % of the post read. How could this be done?

I see that I have two ways to add this code to the website or html or Javascript Snippet. Which way would be the best to add the form to all the posts at the same time?

Thank you