Hi,

Since you are using odoo 14 in sh, it support the custom modules.So you can add the following code in your existing custom module if any or you can create new custom module for this and you can deploy on odoo.sh.

class SaleOrder(models.Model):
    _inherit = 'sale.order'

    def _prepare_invoice(self):
        res = super(SaleOrder, self)._prepare_invoice()
        res['invoice_date'] = self.date_order
        return res

Hope it helps