  The association of FRB 200428 with an X-ray burst (XRB) from the Galactic
magnetar SGR 1935+2154 offers important implications for the physical processes
responsible for the fast radio burst (FRB) phenomena. By assuming that the XRB
emission is produced in the magnetosphere, we investigate the possibility that
the FRB emission is produced by shock-powered synchrotron maser (SM), which is
phenomenologically described with a plenty of free parameters. The
observational constraints on the model parameters indicate that the model can
in principle be consistent with the FRB 200428 observations, if the ejecta
lunched by magnetar activities can have appropriate ingredients and structures
and the shock processes occur on the line of sight. To be specific, a complete
burst ejecta should consist of an ultra-relativistic and extremely highly
collimated $e^{\pm}$ component and a sub-relativistic and wide-spreading
baryonic component. The internal shocks producing the FRB emission arise from a
collision between the $e^{\pm}$ ejecta and the remnant of a previous baryonic
ejecta at the same direction. The parameter constraints are still dependent on
the uncertain spectrum and efficiency of the SM emission. While the spectrum is
tentatively described by a spectral index of $-2$, we estimate the emission
efficiency to be around $10^{-4}$ by requiring that the synchrotron emission of
the shocked material cannot be much brighter than the magnetosphere XRB
emission.
