You're unable to add an embedded link to your invoice email alert using QuickBooks Online (QBO) Advanced plan, Andrea68.

Currently, you can only enter URLs in the Message to customer field or add them as a footer text instead of hyperlinking them to your outgoing invoices. To do this, here's how:

1. Go to the Gear icon.
2. Select Custom form styles.
3. Find your template and click Edit under the Action column.
4. Go to the Content tab, click the footer area, and add the information in the Add footer text or Message to customer field. 
5. Go to the Emails tab and enter the URL in the Message to customer field. 
6. Click Done. 

If you have further questions, don't hesitate to reply to this thread.