coords = SkyCoord(Tx=[100,1000] * u.arcsec, Ty=[100,1000] * u.arcsec, frame=my_map.coordinate_frame)

fig = plt.figure()
ax = fig.add_subplot(projection=my_map)
my_map.plot(axes=ax, clip_interval=(1,99.5)*u.percent)
ax.plot_coord(coords, 'o')
ax.plot_coord(my_map.center, 'X')
plt.show()