Thanks for sending me the test files. The information is set through the exif profile and you can set it like this:
This will only set the description of the image and not the ExifTag.XPKeywords. That one is not yet available in the Magick.NET library. I will try to add the missing tags next week. This means you will need to wait for the next release if the XPKeywords are important to you.
using (MagickImage image = new MagickImage("FA527JN_15_10520_GM.jpg")) { ExifProfile profile = new ExifProfile(); profile.SetValue(ExifTag.ImageDescription, "Magick.NET"); image.AddProfile(profile); image.Write("\test.jpg"); }