Select the face of the outer conductor ( or inner conductor) of it and I placed the WG port using this pick. The signal in a coaxial line is defined by the E field in between the two conductors, that means that the voltage inside the inner conductor is 0 (pec) and the voltage inside the outer conductor is also 0.However there is a difference in voltage BETWEEN those conductors. Therefore, to create a wave in this line you have to create a different voltage between the outer an the inner conductor, so the wg port should cover at least a portion of both conductors