Wis. Stat. § 139.096: Failure to file.

If any taxpayer required to file any return fails to do so within the time prescribed, the taxpayer shall, on the written demand of the department, file the return within 20 days after the department sends the demand and at the same time pay the tax due on its basis. If the taxpayer fails within that time to file the return, the department shall prepare the return from the department’s own knowledge and from the information that the department obtains and on that basis shall assess a tax that the taxpayer shall pay within 10 days after the department sends to the taxpayer a written notice of the amount and a demand for its payment. In any action or proceeding in respect to the assessment, the taxpayer has the burden of establishing the incorrectness or invalidity of any return or assessment made by the department because of the failure of the taxpayer to file a return.