I am sorry, had a typo in code. I have corrected it is last reply. please use that code.

To add a menu button with unique class add following code in your theme’s (child theme’s if any) functions.php

add_filter( 'fep_menu_buttons', function( $menu ) {
    $menu['cus_button'] = array(
        'title'		=> __( 'Edit', 'front-end-pm' ),
	'action'	=> 'edit',
        'class'         => 'fep-button your_unique_class',
	'priority'	=> 15,
    );
    return $menu;
});