Fabric for hanger. Here it is white which works okay for white or other light colors. However choose color depending on the color of the towel or your taste.
Cut out a piece of fabric so that it covers each side on one of the corners of the towel.
Sew the fabric into the towel following the excisting line.
Cut off sewing thread.
And the extra fabric.
And now it can easily hang.
You can hardly see the sewing thread on the back.
Example white sewing thread on white towel.
Hang it :-)
Example with sewing fabric on in the middle of the towel instead of the corner. Do the one you prefer.