Nice to have you join this thread, @Basking in the Glow.

I'll be more than happy to guide you on how to add a Purchase Order (PO) number to your invoice. 

You'll want to customize your invoice template to have a PO field. Then, open the PO and manually copy and paste the number to your invoice. Here's how.

1. Open your invoice in QuickBooks Desktop (QBDT).
2. Go to the Formatting tab and select Customize Data Layout.
3. Check the box for the P.O. No. field under the Header tab, and click OK. 
4. Open your PO, then copy and paste the PO number to your invoice. 

Another way is to utilize a third-party app that lets you automatically link your PO no. to your invoice. Just hover to the App Center or visit our Apps for QuickBooks Desktop page for reference. 

Once your customer pays their invoice, you can refer to this link on how to seamlessly record it: Record an invoice payment.

You can always count on me if you need more help managing your PO and invoices in QBDT. Simply tag me in your reply and I'll take care of them for you. Keep safe and have a wonderful day ahead!