The Br- ion has an extra electron compared to the neutral Br atom, so the electron configuration for Br- is as follows:
1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^10 4p^6
Alternatively, you could write it as the noble gas configuration for krypton (Kr) with an extra electron: [Kr] 4d^10 5s^2 5p^6.