Is it possible to put PO Numbers manually on each Purchase Invoices while I am not using Quickbooks's PO system?

I appreciate your posting here in the Community, Tejas.

I'm here to address them and show you how we can achieve your goal

regarding your concern about putting Purchase Order (PO) Numbers on your invoices despite not using the QuickBooks PO system.

QuickBooks Online (QBO) offers various features you can use along with your business, one of which is customizing your invoices. Furthermore, let's personalize your invoice to have the PO Numbers. We'll create a Custom field and then name it as PO Number. Then, you can manually input your PO number without using the PO system.

Here's how: 

1. Go to Gear Icon, then select All lists.
2. Choose Custom Fields, then click on the Add field tab.
3. On the box under the Name, type in PO Number.
4. Mark checked that box beside the All Sales Form to apply the custom field in your invoice. Note: You can also toggle on the print form so that the custom field will appear on the printed invoice.
5. Once done, click Save.

Moreover, to gain more understanding about adding custom fields to your sales forms, I recommend scanning through this article: Add custom fields to sales forms and purchase order orders in QuickBooks Online.

After sending invoices to your customers, you'll have to receive and record their payments. You can utilize this article to learn how to do it in your QBO: Record invoice payments in QuickBooks Online.

Let me know if you have additional questions about adding a PO number to your invoices in QBO. I'll be here to answer them any time.