Prepend a CTA to your RSS feed body/content or text to your subject line

functions.php

<?php

// prepend a call to action to your email body by adding before your the_content()
function prepend_rss_cta($content) {
	$cta = ' <p><i>Want to share or link to this post? It lives here:  <a href="' . get_the_permalink( get_the_ID() ). '">' . get_the_title( get_the_ID() ) . '</a>.</i></p>';
	$content = $cta . $content;  // switch cta and content to append (not prepend) your CTA
	return $content;
}
add_filter('the_content_feed', 'prepend_rss_cta');

// prepend some text to your subject line by adding it to before your post title
function prepend_rss_title_subject_line($title) {
	$acronym = '[TEI] ';
	$title = $acronym . $title; 
	return $title;
}
add_filter('the_title_rss', 'prepend_rss_title_subject_line');