Sample code to add Facebook Messenger as a sharing button to Scriptless Social Sharing.

functions.php

<?php

add_filter( 'kses_allowed_protocols', 'prefix_allow_messenger' );
/**
 * Add Facebook Messenger to the list of allowed URL protocols in WordPress.
 *
 * @param $protocols
 *
 * @return array
 */
function prefix_allow_sms( $protocols ) {
	$protocols[] = 'fb-messenger';

	return $protocols;
}

add_filter( 'scriptlesssocialsharing_messenger_url', 'prefix_add_fb_messenger', 10, 3 );
/**
 * Add SMS to settings/allowed sharing buttons
 *
 * @param $url
 * @param $button
 * @param $attributes
 *
 * @return mixed
 */
function prefix_add_fb_messenger( $url, $button, $attributes ) {
	$app_id = ''; // enter your fb app ID here
	if ( ! $app_id ) {
		return $url;
	}

	$url = add_query_arg(
		array(
			'link'   => $attributes['permalink'],
			'app_id' => $app_id,
		),
		'fb-messenger://share/'
	);

	return $url;
}

add_filter( 'scriptlesssocialsharing_networks', 'prefix_scriptless_networks' );
/**
 * Add FB Messenger to settings/allowed sharing buttons
 *
 * @param $networks
 *
 * @return mixed
 */
function prefix_scriptless_networks( $networks ) {
	$networks['messenger'] = array(
		'name'  => 'messenger',
		'label' => __( 'Facebook Messenger', 'scriptless-social-sharing' ),
		'icon'  => 'f39f',
		'color' => '#333', // change this to whatever color you want the button to be
	);

	return $networks;
}