Sample code to add the Print Friendly service as a scriptless sharing button

functions.php

<?php

add_filter( 'scriptlesssocialsharing_networks', 'rgc_scriptless_networks' );
/**
 * Add Print Friendly to settings/allowed sharing buttons
 *
 * @param $networks
 *
 * @return mixed
 */
function rgc_scriptless_networks( $networks ) {
	$networks['print'] = array(
		'name'  => 'print',
		'label' => __( 'Print', 'scriptless-social-sharing' ),
		'icon'  => 'f02f', // the glyph for the Font Awesome icon
		'color' => '#121212', // change this to whatever color you want the button to be
	);

	return $networks;
}

add_filter( 'scriptlesssocialsharing_print_url', 'rgc_add_print_friendly_url', 10, 3 );
/**
 * Add the print friendly URL to the new print button.
 *
 * @param string $url
 * @param string $button_name
 * @param array $attributes
 * @return string
 */
function rgc_add_print_friendly_url( $url, $button_name, $attributes ) {
	$query_args = array(
		'url' => $attributes['permalink'],
	);

	return add_query_arg(
		$query_args,
		'https://www.printfriendly.com/print'
	);
}