How to include GST% in PO amount?

How to include GST% in PO amount?

Currently when I raise PO, the basic price of the item is used.
However, the GST is not calculated and added along with the basic amount.

I have added a Custom field for GST in the PO but I need Zoho Inventory to calculate the GST amount and add it with the Basic price to give me the final PO price.