Wisconsin does not tax military retired pay. Thrift Savings Plan distributions are a separate thing and are taxed by Wisconsin, because TSP contributions are made with before tax dollars.
There is nothing to apply for. Leave your military retired pay out of the income you report on your Wisconsin return. If Wisconsin tax was withheld from it by mistake, file a return to claim the refund.
Your Form 1099-R showing the retired pay, kept with your tax records.