Marco:
If an old registration card is in the car it may have the plate # on it.
Otherwise, this is likely where you'll have to call or Email to get the requesting info:
http://www.dot.wisconsin.gov/drivers/records.htmLooks like you can copy and paste your request in an Email to them and include the VIN #.
Question:
How will the Portuguese authorities know if you made up a plate #? Will they actually verify with the Wisconsin Department of Motor Vehicles (DMV)?
For the record, each state has their own process regarding the DMV. So, your request may be handled in a completely different manner from one state to another. In fact, the info you request may not be available in all jurisdictions.
Let us know if you need help.