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Create a role profile question called 'Purchase Order'. See the article Profile Questions - Configuring for instructions on setting up profile questions
Configure the appropriate data type, question, default answer, etc.
Add the profile question answer to your invoice template. See the Invoice Template Sample Code article for an example.
Add the profile question to the customer accounts on their overview page
Click the Add button to add a component
Select the profile question just added, for example 'Purchase Order Question'
Enter the Purchase Order number required
Finally confirm that the invoice for that customer (if its already open or create a new invoice) shows the PO number as desired