Hi there, @derek-apigrate.

Welcome to the Community space. I can route you to the correct website specific to QuickBooks Online API concerns.

The developer site is the best support to handle your questions about assigning templates to an invoice. After you've signed up and posted this question, our specialist from this Community will reach out to you and provide you the resolutions you need.

You can use this link to get in touch with them: Intuit Developer Community.

That information should get you on the right path.

If there's anything else I can do for you, please don't hesitate to reach out to me again. I'll be around to help you out.