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To add purchase order numbers follow the general outline provided below. Supplementary articles are availabe available for the specifics in each case.
Create a role profile question called 'Purchase Order'. See the article the Configuring Profile Questions - Configuring for article 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 QuestionNumber'
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