Hi,
this is going to be really complicated, as you don't have the whole booking in the my bookings view (just some details) and you would need to get at least these (semid, uuid) etc.
Code would look something like that:
$link = JRoute::_("index.php?option=com_matukio&view=book&task=cancelBooking&cid=" . $this->booking->semid . "&uuid=" . $this->booking->uuid);
echo " <a border=\"0\" href=\"" . $link . "\" ><span class=\"btn btn-default\" type=\"button\">"
. JTEXT::_('COM_MATUKIO_BOOKING_CANCELLED') . "</span></a>";
But you should add some checks if cancelling is possible!
Cheers,
Yves