How can I add specific capability to custom user role

Question

    $capabilities = array(
        'read' => true,
        'edit_pages' => false,
        'publish_pages' => false,
        'edit_published_pages' => false,
        'delete_published_pages' => false,
        'pages' => false,
        'manage_options' => false,
        'edit_dashboard' => false,
        'customize' => false,
    );
    $existing_role = get_role('str_mngr');
    $admin_role = get_role('administrator');
    if (!$existing_role) {
        add_role('str_mngr', 'Store Manager', $capabilities);
    } else {
        foreach ($capabilities as $cap => $grant) {
            if ($grant) {
                $existing_role->add_cap($cap);
                $existing_role->add_cap('manage-store');
                $admin_role->add_cap('manage-store');
            } else {
                $existing_role->remove_cap($cap);
            }
        }
    }

I'm using this code snippet.How can I add specific capability to custom user role I want to remove all capabilities for Store Manager and want to add a single capability to see and manage the menu 'manage-store'.