Hi there, Rani1. 

Thanks for dropping by the Community, I'm happy to help. If you're trying to add a message to an invoice and you're using QuickBooks Online, follow along below:

1. Go to the Sales menu.
2. Choose the Invoices tab.
3. Locate and open the invoice, you wanted to add the message. 
   
4. On the invoice page, go to the Message on invoice section.
5. Type in the purchase order number and the client's name.
6. Once done, click Save.
7. Don't close the invoice and click the Print or Preview option to view the form. 
   

Once done, print the invoice again, and check if you can see the number already.

There’s also an option for you to personalize your messages for each sales form. Please check this article for further details: How to Set Up or Change Customer Messages.

If you have any other questions, feel free to bring them to our attention down below. Thank you for your time and have a nice afternoon.