  At least some fast radio bursts (FRBs) are produced by magnetars. Even though
mounting observational evidence points towards a magnetospheric origin of FRB
emission, the question of the location for FRB generation continues to be
debated. One argument suggested against the magnetospheric origin of bright
FRBs is that the radio waves associated with an FRB may lose most of their
energy before escaping the magnetosphere because the cross-section for $e^\pm$
to scatter large-amplitude EM waves in the presence of a strong magnetic field
is much larger than the Thompson cross-section. We have investigated this
suggestion and find that FRB radiation traveling through the open field line
region of a magnetar's magnetosphere does not suffer much loss due to two
previously ignored factors. First, the plasma in the outer magnetosphere ($r
\gta 10^9 \ $cm), where the losses are potentially most severe, is likely to be
flowing outward at a high Lorentz factor $\gamma_p \geq 10^3$. Second, the
angle between the wave vector and the magnetic field vector, $\theta_B$, in the
outer magnetosphere is likely of the order of 0.1 radian or smaller due in part
to the intense FRB pulse that tilts open magnetic field lines so that they get
aligned with the pulse propagation direction. Both these effects reduce the
interaction between the FRB pulse and the plasma substantially. We find that a
bright FRB with an isotropic luminosity $L_{\rm frb} \gta 10^{42} \ {\rm erg \
s^{-1}}$ can escape the magnetosphere unscathed for a large section of the
$\gamma_p-\theta_B$ parameter space, and therefore conclude that the generation
of FRBs in magnetar magnetosphere passes this test.
