Hey there, @Jun3.

The option to add another field to your invoice email message is unavailable in QuickBooks. As suggested above, you have to manually enter the specific customer/company name in the subject line in the Custom Form Style Settings.

Aside from the steps provided above, there is an easier way to edit the invoice's subject line. You can do it before sending the transaction to your customers. Just directly change the Subject details in your email invoice before clicking Send and close.

I can see how this option would be helpful for you and your business. Therefore, let me take note of this as a suggestion to improve your QuickBooks experience.

In the meantime, I recommend visiting our Firm of the Future site. This is where we share recent happenings and future developments, such as updates to newly added features.

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