D.C. Code § 21-110

Before original or ancillary letters of guardianship are issued, the person designated, if a nonresident of the District of Columbia, shall file in the office of the Register of Wills an irrevocable power of attorney designating the Register of Wills and his successors in office as the person upon whom all notices and process issued by a competent court in the District may be served, with like effect as personal service, in relation to all suits, matters, causes, or things affecting or pertaining to the estate in which the letters are to be issued. The Register of Wills shall forthwith forward by registered or certified mail to the address of the guardian, which shall be stated in the power of attorney, all notices or process served upon the Register under this section.
If the person fails to file the power of attorney within 10 days after the entry of the order of appointment, the order shall stand revoked, and he shall forfeit all rights to the office.