Adding a Dokan Submenu

Hi, I’m trying to add submenu to Dokan in my plugin. However, I couldn’t add it. Where am I doing wrong? Can you help me ?

class Admin { 
    public function __construct() { 
        //add_action( 'dokan_admin_menu', array( $this, 'refund_info_template' ) );
        add_action( 'admin_menu', array( $this, 'refund_info_template' ) );
        
    }

    public function customer_relationship() {
        add_submenu_page( 'dokan', __( 'Customer Relationship ', 'cr' ), __( 'Customer Relationship', 'cr' ), 'manage_options', 'customer-relationship', array( $this, 'customer_relationship_menu_context' ), 5);
    }
    public function customer_relationship_menu_context() {
        echo 'defined';
    }
}