Apart from info record tax code can be defaulted in the Purchase order by maintaining the following:
Have Condition type MWST - Input tax
Create a condition table with the Keys Combination Plant/Vendor/ Company Code in M/03
Assign the Condition table to the access sequence MWST in M/07.
Ensure that the Condition type MWST have access sequence MWST (M/06) and maintained in the MM Pricing Procedure in M/08.
Maintain the Condition Record in MEK1 & enter tax code with Plant, Vendor, and Company Code and save.
Now create the Purchase order with combination Plant/Vendor/ Company Code, the tax code will be defaulted.