Your commands translates to the example below. But it does not work correctly due to a bug in ImageMagick. This will be fixed the next release of Magick.NET.
using (MagickImage image = new MagickImage("cyclops.png")) { image.Alpha(AlphaOption.Set); // -alpha set image.ColorFuzz = 20; // -fuzz 20%// You don't need to specify -channel RGBA image.FloodFill(MagickColor.Transparent, 0, 0, new MagickColor("#FFF")); // -fill none -floodfill +0+0 white image.Write("cyclops_flood_1.png"); }