Promotions into the Senior Foreign Service shall be recommended by selection boards only from among career members of the Service assigned to class 1 in the Foreign Service Schedule who request that they be considered for promotion into the Senior Foreign Service. The Secretary shall prescribe the length of the period after such a request is made (within any applicable time in class limitation established under
section 4007(a) of this title) during which such members may be considered by selection boards for entry into the Senior Foreign Service. A request by a member for consideration for promotion into the Senior Foreign Service under this subsection may be withdrawn by the member, but if it is withdrawn, that member may not thereafter request consideration for promotion into the Senior Foreign Service.