How to update a site column field using workflow variable

Question

First let me say I am new to sharepoint workflows. Having said that, here's my question:

I need to create a workflow that fires when a document is added to [po request] document library. The workflow is a basic approval of a purchase request. If the request is approved, the task approver will approve the task. I would also like for the task approver to provide a Purchase order # and have the workflow update a column in the document library with that Purchase order #. I was thinking of doing this by defining a workflow variable and then using that variable in a custom task to receive the purchase Request number from the user. Then I would like to be able to update a site column [PO Number] with the value of the purchase request number. Any ideas on how to do this?

Answer 1

When you create a task in the workflow, it creates a variable that stores the list item ID of the task. After the task step, you'd want to use that variable to look up the item in the task list, store the PO# in a variable and then Update the current item PO# to that variable.