Is it possible to fetch the "reference" field from the invoice to the email header and email body?

Yes, it's possible to fetch the reference field in your invoice form, @justin. And happy to give the detailed steps and screenshots to guide you accordingly. 

I'll make sure you can add a PO number field to your invoice forms on QuickBooks Online. This way, you can easily view the invoice with a purchase order enclosed.

You'll need to customize the invoice to remove the reference field from the form. Then, create a PO No. box by following these steps:

1. Go to the Gear icon, then choose the Custom fields. 
2. Click the Add custom field button. 
3. Enter PO Number. and check the box beside All Sales forms.
4. Turn on the radio button in Print on the form.  
5. Hit Save.
6. If you see the Reference field in the list, click the small drop-down arrow and select Make inactive. 

For more information, check out this link: How to add custom fields to sales forms and purchase orders.

After that, create a sample invoice to see how the custom field looks like in the form. 

1. Go to the +New and select invoice. 
2. The custom field you created will appear in the form. 
3. You can click the Gear icon to customize some columns you want to show in the sales form. 

Additionally, you have an option to create a personalized invoice template by going to the Gear icon and selecting Custom form styles. To find out the complete instructions, see this link: Customize invoices, estimates, and sales receipts in QuickBooks.

Did you know that QuickBooks can generate invoice reminders and recurring transactions/payments? For the complete details, see these articles: 

• Send invoice reminders automatically or manually in QuickBooks
• Create recurring transactions in QuickBooks Online

You can always let me know if you have any more questions about QuickBooks. I'm a few clicks away to help you. Keep yourself safe, Justin. 

We already have the custom field "PO". I'm trying to capture that information from the invoice to the email template. This way when we send invoices to customers, in the subject header of the email.  

Hello @justin-advetage-,

I appreciate you for getting back to us with some clarifications. Allow me to chime in and share some information about the email subject line in QuickBooks.

The ability to include another field aside from the invoice number is unavailable. As a workaround, you can enter the information manually on the invoice's subject line before sending the transaction to your customers.

You can directly change the Subject details in your email invoice before clicking Send and close. Check out this screenshot for your visual guide:

I've also attached an article you can use to personalize the greeting that is emailed with sales forms or print a message on the statement: How to set up or change customer messages.

The Community always has your back, so please let me know if you have any other questions about emailing an invoice. I'll be more than happy to help. Keep safe.