Input the color excess E(B-V).
The magnitude (or flux, or line flux) given by the user is assumed to be the OBSERVED value, and hence already suffers this reddening.
The color excess value is used to adjust the absorption from that of the input magnitude (e.g. V) to the absorption at the wavelength of the requested filter (e.g. F439W).
Note that the visual extinction, Av, is roughly given by Av = 3.1 E(B-V).