When click "add a line" button on sale.order.line as example, i want to show a warning message, but then still lets a new line show up, how it that possible?

In odoo 15 by default, this is not possible, i've try this but not working.

Please help, thanks.

@api.onchange('product_id')
    def onchange_product_id(self):
		// my code
		return {
                    'warning': {'title': ('Warning'), 'message': ('my message'), },
                }