functions.php

<?php

// Register newsletter widget area
genesis_register_sidebar( array(
	'id'		=> 'newsletter',
	'name'		=> __( 'Newsletter', 'custom-theme' ),
	'description'	=> __( 'This is the newsletter section.', 'custom-theme' ),
) );

// Add the newsletter widget after the post content
add_action( 'genesis_after_post_content', 'custom_add_newsletter_box' );
function custom_add_newsletter_box() {
	if ( is_singular( array( 'post', 'page' ) ) )
	genesis_widget_area( 'newsletter', array(
		'before' => '<div id="newsletter">',
	) );
}

style.css

/*
Newsletter
------------------------------------------------------------ */

#newsletter {
	background-color: #222;
	color: #fff;
	line-height: 1.5;
	padding: 32px;
	padding: 2rem;
	text-align: center;
}

#newsletter p {
	margin-bottom: 24px;
	margin-bottom: 1.5rem;
}

#newsletter input {
	width: 50%;
}