How do I add this to end of each post?

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At the bottom of my “about me” page, you will see that I have created a div with an image and a mailchimp form for email sign up. I want to put the same at the end of each single post. However, all of the css is included in the email form.

I under stand that I need to add

function emailfeature_after_post_content($content){
if (is_single()) {
	$content .= '??????';
}
	return $content;
}
add_filter( "the_content", "emailfeature_after_post_content" );

What I am not clear on is what goes where the question marks are. Do I insert the entire code as I have on the “about me page” (I think that would break it), do I insert “div class=emailfeature” or something else. (still learning)
Then there is the issue of my related posts. I want this signup to come before them, but without causing a conflict. It’s a mess, I know. But is there a way to do this easily, and without a plugin.