I have a custom invoice set as the default form however when I try and create an invoice it does not use this form. How can I change it so it is the form used?

Let me share the steps in customizing the invoice form, @GET PT. 

To begin with, by setting the customized invoice, we need to make it as default in your QuickBooks Online (QBO) account. Doing this will make your moving forward transactions set to your preference. 

Here's how: 

1. Go to the Settings.
2. Select Custom Form Styles.
3. Locate the desired template and choose the drop-down arrow.
4. Select Make Default.

About adding a field, such as a person's reference or healthcare number, we can add them in the custom fields. 

Here's how: 

1. Go to the +New icon, then select Invoice.
2. On the invoice page, click the Gear icon.
3. Go to the Custom fields tab.
4. To create a custom field, select + Add custom field. Fill out the info about the field, then select Save.

We can repeat these steps for health care numbers and person's reference.

In addition, you'll want to check this article to learn how to convert a quote into an invoice in QuickBooks Online: Convert a quote into an invoice in QuickBooks Online. 

I'm always available in the Community if you need additional assistance managing your sales forms. Stay safe.