Hello there, @Rueben1.
 

I'll be glad to walk you through how to change the subject line to pull in from the field instead of using the invoice number.
 

You can manually edit the subject line upon sending the invoice or by going to the Custom Form Styles.
 

Here's how:
 

1. Click the Gear icon and choose Custom Form Styles.
2. Locate the Standard or Master Template and click Edit.
3. Go to the Emails tab.
4. From the Subject field, add the line you want to show in your email subject line.
5. Then select Done.

I've also added a Screenshot for visual reference.

To see steps for adding fields as well as changing the colour scheme, you can click here.
 

Please let me know in the comment section if you want to know about Custom Fields in QuickBooks Online. I'll be happy to assist you. Enjoy the rest of the day!