Finally I managed to find the answer myself. The link between purchase order and sales order is not created on the level of the orders, but on the level of the order lines. Actually, in our case where we subcontract a service we sell, by setting the product (service) as being outsourced, whenever a SO is validated, this triggers the creation of a PO for the respective supplier.
By using Studio, you can modify the PO order lines section in the form view. Add the field "Sale_order_id" to it, and make sure this field is not 'read only'. (this can be achieved by adding a domain to the 'read only' flag (like e.g. Id is not set). And for security reasons, make sure this field is only visible to certain user groups.
Now you can edit / add / link the purchase order line to the sales order it should be linked to. In addition, you can even add the sales order line on the above mentioned list view, so that you can back link the purchase order line to the original sales order line.
By going back to the sales order, you will find that the smart button in the top corner now links to the adapted PO.
Glad I could help myself (and others.)
You can set vendor in the product, and set buy routes and Make to order, this will generate a purchase order whenever you make sales order for that product