Thanks for getting back to the thread and clarifying your concern, @mikeincairns.
For now, using an HTML signature on your emailed invoices is unavailable in QuickBooks Online (QBO). However, you can create your own customize form styles outside QuickBooks and import it into the program. Then, use your design to add details to your invoices.
First, let's make sure to turn on the Import Style. Just go to Settings and select QuickBooks Labs.
Look for the Import Style section and turn it on. Then, click Done.
Go back to Settings and select Custom form styles.
Click the New style ▼ drop-down menu, then Import style.
Select Invoice and click Next.
If it's the first time you import your sales form, click Download import guide and check for the invoice template document sample.