Add Custom Button to Horizontal Floating Share Bar

Since MashShare ShareBar Add-On version 1.2.1 you can use a custom filter function which you can add at the bottom of your theme’s functions.php

add_filter( 'mashbar_networks', 'prefix_custom_networks_sharebar', 10 );

function prefix_custom_networks_sharebar( $networks ) {
    $networks_more = array(
    	'custom_button' => array(
		'url' => 'http://www.yourwebsite.com',
		'status' => 1,
		'name' => 'A Custom Name'
    	)
    );
    return array_merge( $networks, $networks_more );
}

CSS to specify a custom background colour e.g.:

.mashicon-custom_button {
    background-color #000;
}