How to key in invoices with desired GST tax codes.

First, head to GST > GST Settings and choose what tax codes you will be using in your business (when issuing invoices). Save the changes.

Then, go to "Product/Services > Find the product > Edit", change the Output tax code to the desired tax code. When issue invoice, the product will have the tax code you had chosen. Similarly, you can change the Input tax code too.