How to increase Prophet forecast because of a new external event

Hello,

I have developed a prophet model to forecast vehicle transportation cost. But due to some external event the future transportation cost is bound to increase . How will I incorporate this new change so that my forecast also increase according to this event.

Please note: - This event is a government policy/law change for which the price will increase and it is happening for the 1st time. So I cannot add a external regressor as there is no historical information on this (to train the model).

Thanks in advance.