Car park ticket for motorbikes
According to the German Road Traffic Act/ Highway Code there is generally no exception for motorcyclists even if there are problems displaying it properly.
The easiest way for displaying a valid parking ticket is to stick it with adhesive tape on the headlight. If available, keep the ticket as evidence. A small plastic bag containing the ticket attached to the handlebar is another option. Even it is a parking space for motorcars there is no bonus for motorbikes so that every motorbike must have its own pay and display ticket.