Hi,

Try declare a Many2one field in the purchase order and pass the id of the sale order to that many2one field, since you are using the above module, you can pass value using the function action_create_purchase_order in the model 'create.purchaseorder' .

In purchase order

sale_reference_id = fields.Many2one("sale.order",string="Sale Reference")

In create.purchaseorder

in the function action_create_purchase_order update the following part like below.

		res.create({
			'partner_id' : self.partner_id.id,
			'date_order' : str(self.date_order),
			'order_line':value,
			'origin' : sale_order_name,
			'partner_ref' : sale_order_name,
			'sale_reference_id': so.id
		})

Regards