It's nice to have you in the Community, @Wasapi. Allow me to share some details about the placeholder you can use.

Placeholders in QuickBooks Online follow the same pattern. For example, the invoice placeholder is [Invoice No.]. All other transactions that you send out to your customers will have this same pattern.

You can also create a different default email message for a different sales form. Here’s how:

    1. Go to the Gear Icon, then click Account and Settings.

    2. Select Sales, then click the pencil icon in Messages.
    3. Click the sales form you want to use.
    4. You can also enter the placeholder in the Email Subject line below.
    5. Click Save and Done.

Check out this article for more details on how to customize and design your invoices: Customize invoices, estimates, and sales receipts in QuickBooks Online.

Please let me know if you have any questions about QuickBooks. I'll be right here to help. Wishing you the best.

Can we map custom fields yet?

Can custom fields be mapped yet?

Hello there, aainc.

I’m here to assist you with any QuickBooks concerns. I just need some more information to help me get on the same page.

May I know if you’re trying to map the custom fields on imported custom sales forms? Are you referring to the ones found on the Custom fields page?   

I appreciate any details you can provide. I’m looking forward to your reply. Have a great rest of the day.

I have custom fields on my invoices. Similar to how [Invoice] maps to the invoice number and shows in the subject line of the email, I would like to use [Custom Field 1] to do the same

 Hi there, @aainc.

Being able to add another custom field to the invoice email would be great. I can see how this would be useful. Rest assured that I'll relay this message to our team in concern here on my end and add your vote for this one.
 
You can also share your thoughts and ideas through your account. Our product development team reviews all the feedback we receive to ensure we’re meeting the needs of our customers. 

Here's how:

1. Select the Gear icon at the top, then Feedback.
   
2. Enter your comments or product suggestions then select Next.
3. You'll be provided a list of suggested Help articles related to your comment. You can read through them or select Skip and send message.
4. From the drop-down, choose the appropriate category, then select Send message.

For your reference, check out this link: How to Submit Feedback.

In the meantime, you can manually enter the information in the subject line before sending the transaction. 

If you have any other questions, just let me know and I'll be happy to help. Thanks again for reaching us and have a great day.

@aainc Have you heard about any updates yet?

QB? When can please add other mappings in the subject line of emails for sending invoices???

Please. This is such a pain. 

Every customer has requirements.

I vote for adding a custom fields to emails as well!! When customers have several invoices, it would be nice to have the custom field appears so they know which projects the billings are for.