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.