Hello there, eekie75.

I can see the benefit of including the purchase number in the email subject when you send it to your supplier. However, the option to include it in the email subject when sending it to your supplier is unavailable. 

In the meantime, you can manually add the PO number in the subject column. Just change the subject details to include your purchase number before clicking the Save and Close button.

While it's not currently possible, I recommend submitting a feature request directly to our product developers. This will allow them to assess the feasibility of incorporating it into future updates.

Here's how:

1. Click the Gear icon⚙, then Feedback.
2. Enter your comments or product suggestions. 
3. Then select Next to submit feedback.

Once done, your valuable suggestion goes to our Product Development team to help improve your QBO experience. 

Additionally, let me share these links that will guide you in sending and adding an accepted purchase order for your supplier:

• Create and send purchase orders in QuickBooks Online
• Add purchase orders to expenses, bills, or cheques in QuickBooks Online

If there's anything else you need help with, please let me know by commenting below. I'm always here to answer any questions you may have. Take care.