Please find code here :-

For Example -
Note - Check Indentation of the code after pasting the code.

- Create an automated action by adding
name, model( Task (project.task) as per your requirement),
trigger(update as per your requirement), Trigger fields (Purchase Order (project.task) - Purchase Custom Field as per your requirement), Action To Do ( Execute Python Code ).

- In the Python Code Below Paste this code:-

for rec in records:
for task in rec.project_id.task_ids:
task.write({
'purchase_order_id':rec.purchase_order_id,
})