Service Tax on PO Matched AP Invoice

Hi,

1) I have created a Standard PO with out Service TAX.

2) I have Created invoice with Qucik PO Match option.

3)Is there any option to Enter Service TAX on PO Matched Invoice?

4)After Validating I am able to have the TDS Correctly.But not able to calculate the Service TAX on AP Invoice after PO Matching.I don't find the Option to enter Service Tax on PO Matched Invoice.

5)I want to calculate and apply the Service TAX and TDS on PO Matched Invoice.Even If i miss to enter the Service Tax at PO Level.

6)And also Can we apply the TDS at PO Level?with out Claculating at AP invoice Level