Unless the US doc is also licensed to prescribe in Germany and has the equipment to transmit a German prescription, this will not work.
Your hotel, the US Consulate in Frankfurt, or perhaps a friend / colleague can recommend a local doctor who can speak with your US doc and, depending on the med. prescribe it.
Depending on what the medication is, it may be possible for a friend/relative in the US to overnight the meds to you, presuming that they can get access.
All of this is dependent on the specific medication.