Yes, you could and should have said no to the test. There was no reason to give you the test since you weren't driving. However, as an underage drinker you may have a problem with your license now that you've taken the test and I'm fairly sure there is a zero tolerance enforcement in your state.
Speak to an attorney locally right away.
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Yes, you could have said no. But if there was other evidence you were drinking (i.e., the odor of alcohol in the car, open containers, a friend said you were drinking, etc.), you likely would have been given an underage drinking ticket anyway.
Given the facts contained in your post, the officer did not have enough reason to ask you for a PBT. But, we may not have all the facts.
In addition to attorney Missimer's answer above, I would note that you may have a possible suppression issue based upon an unlawful stop. The WI window obstruction statute is rather vague and some vehicle stops based upon something hanging from the rear view mirror have been successfully challenged in the past. As a passenger in a vehicle you have legal standing to challenge the constitutionality of the vehicle stop by police.