Go to the side navigation bar and select "Customers" from the options.

Choose the specific customer you want to set the Billing Template for.

Scroll down on the right-hand window to locate the "Billing & Contract" tab and click on it.

Select the "Billing & Invoice" tab from the available options.

Next to the invoice number, you'll find a dropdown menu allowing you to select either 'manual' or 'auto-generated' options.

If you select Auto-Generate, two fields will appear. The first one offers a dropdown where you can choose from options like "INV" and your customer code. The second field is for entering a custom prefix with alpha-numeric values, and the auto-generated value will be incremented after the custom values you've entered.

If you choose manual, you can enter the invoice number at the time of creating the invoice, in the editable invoice number field.