Hi there, @sales216.
You'll have to create a GST again then make sure to select the IGST tax once you create an invoice. Let me walk you through the steps on how to create IGST tax. Here's how:
I've also added an article on how to apply IGST to your invoice.
If you have additional questions, please don't hesitate to leave a message in the comment section. I'll be happier to help. Take care, and have a good day.