Add content to recent post widget. In this case just before after_widget parameter.

dynamic-sidebar-params.php

<?php
/**
 * Add content to Recent Posts Widget.
 *
 * @since 1.0.0
 *
 * @param  array $params Array of widget arguments.
 * @return array Modified 'after_widget' argument.
 */
function prefix_add_content_to_recent_post_widget( $params ) {
	
	if( ! is_admin() && false !== strpos( $params[0]['widget_id'], 'recent-posts' ) ) {
		$params[0]['after_widget'] = 'Add you content' . $params[0]['after_widget'];
	}
	
	return $params;
	
}
add_filter( 'dynamic_sidebar_params', 'prefix_add_content_to_recent_post_widget' );