How can we make a Taxcode as default on purchase order item detail screen

A1: Use condition type NAVS with access sequence 0003 to default the taxcode in the purchase order item using conditions.
A2: Go to IMG.
Materials Management -> Purchasing -> Purchasing order processing ->define screen layout at document level.
Search for the transaction ME21.Double click on ME21. Then search for TaxCode. It is advisable to configure also the info record. The tax code should be a required entry, and whatever you specify here will be the default value in the Purchase order.
Materials Management -> Purchasing -> Purchasing info record -> define screen layout.
When you get here you search for the transaction ME12 ,double click here; search for Input VAT indicator.