At the time the award is offered, you will have the opportunity to make a final decision on the school you'd like to attend. It is also possible to transfer your Hertz Fellowship to a different participating school during the course of your graduate work if this becomes desirable. At that time, you may also petition for your desired school to be added to the Participating School list, if it is not already included. (See the answer to the above question for the procedure involved.)
Note that Hertz Fellowships can only be used at schools that are currently participating or become participating schools.