Posting Purchase Invoice for Imports Under India GST Localization

I am trying post Purchase Invoice for my imports, but stuck with the whole workflow here. This is what I have done so far:

1. Created a vendor with GST Vendor Type as 'Import', leaving the GST Registration Number blank.
2. Created a Purchase Invoice with relevant details in the line including GST Assessable Value.

But when I try to calculate GST (IGST) would not calculate. As per a previous thread in the community, I tried creating a GST Posting Setup with State Code as blank, but it wouldn't accept a blank State Code.

Can someone explain to me how Posting of Purchase Invoice for Imports Under India GST Localization workflow.