On a motorbike the sticker has to be affixed to a non-removable part of the bike.
Popular choices are the frame or the front fork. Other options include tank or wind screen, as long as they are permanently mounted meaning non-removable.
Important: Once the sticker is attached, do not try to remove it, this will destroy the sticker and a new sticker has to be purchased. Do not use any kind of adhesive or tape, this also leads to the sticker becoming invalid. Same penalties apply as for driving without a vignette.
Related articles:
Can I remove and reuse a physical Swiss vignette (Sticker)?
Where has the Swiss sticker to be placed on windshields of cars, campervans and caravans?
Where has the Swiss sticker to be placed on the trailer?