This is how…..

convert source.jpg -gravity SouthEast -pointsize 16 -font TinyUnicode-Medium -fill "#fffdc3" -annotate +10+10 "watermark text" destination.jpg

More readings…

• https://www.imagemagick.org/Usage/annotating/#text_position
• http://stackoverflow.com/questions/26658415/imagick6-gravity-southeast-while-using-draw-image-failed