How do I add retention to an invoice? this retention is in relation to the monies retained by contractor which can then be released upon completion of the job

How do i Add to apply a retention to an invoice?

In my country, the law mandates 2% retention on the amount of the invoice. Additionally, there is a 1% on the amount of the invoice in favor of the municipality.

That is to say that the invoice I issue is taxed with 3% withholdings. How do I register these retention ?, attached sample.  This Sample is for 2% of retention a favor of government.