We appreciate you posting here in the Community, @Rozmore13.
Could you please elaborate on what you mean by embedding an invoice to an email in Custom Form Styles?
It would also be great if you can provide us with a direct link for the ProAdvisor module that you're referring to or share a screenshot of the instruction from the training guide. This will help us verify the issue further.
In the meantime, I recommend you check out this to learn more about how to personalize and add specific info to your sales forms: Customize invoices, estimates, and sales receipts in QuickBooks Online.
Additionally, I'll also add this link where you can search for articles about the company's income and customers in QuickBooks Online (QBO): QBO Sales and Customer's Help Articles.
Feel free to provide us with more details of your concern by adding a reply below. The Community team always has your back. Have a good one.