Is there a formula you can enter in the purchase order's subject line, so the PO# will show when sending a PO out?

Hi laura-ashbridgee,

QuickBooks Online offers tons of great automation features to make it easier for you to run your business. I'll be happy to share more on this.

It sounds like you're already familiar with these following steps to display the number field on the Purchase Order form.

1. Select + New.
2. Choose Purchase Order.
3. Click on the gear icon at the upper right and select the Custom transaction numbers feature.

At this time, there's no feature nor formula available to automatically include the PO number as part of the email subject line. To have the PO number in there, you'd have to enter it manually. I can see how having an automation option for this would be useful for your business, and I encourage you to submit feedback directly to our engineers to share your thoughts on this. We're always looking at ways to improve our product, and your opinions help a lot in that.

Feel free to stop by any time if you need anything else. 

Have a great day!