Sure, I can help with writing a Python code that implements a POWL model for a Purchase-to-Pay process in pm4py. The Purchase-to-Pay process usually consists of Requisition, Purchase order, Goods receipt, Invoice receipt, and Payment. Here, we will make a sequential workflow with these activities. The activities are partially ordered where every next activity depends on the completion of the previous one.

Below is the Python code necessary to implement the POWL model for the Purchase-to-Pay process:

```Python
import pm4py
from pm4py.objects.powl.obj import Transition, StrictPartialOrder

# define activities in the purchase-to-pay process
requisition = Transition(label="Requisition")
purchase_order = Transition(label="Purchase Order")
goods_receipt = Transition(label="Goods Receipt")
invoice_receipt = Transition(label="Invoice Receipt")
payment = Transition(label="Payment")

# define process order
p2p_process = StrictPartialOrder(nodes=[requisition, purchase_order, goods_receipt, invoice_receipt, payment])

# define order of activities 
p2p_process.order.add_edge(requisition, purchase_order)
p2p_process.order.add_edge(purchase_order, goods_receipt)
p2p_process.order.add_edge(goods_receipt, invoice_receipt)
p2p_process.order.add_edge(invoice_receipt, payment)
```

Please note that workflows in real-life are usually more complex and involve choices, loops, and parallelism, which is not reflected in this simple sequential workflow. This is just a simple demonstration to help you get started with using pm4py to model workflows.