Because it is inherited through the Y chromosome, only males will receive a paternal haplogroup assignment. If a male relative such as a father, brother, paternal uncle or paternal male cousin is genotyped then a woman can infer her own paternal haplogroup information from his. For women that have direct paternal relatives genotyped at 23andMe, they can inherit the Paternal Line assignment from one of these relatives.
In order to link a paternal haplogroup to your profile, first make sure that you’re sharing genomes with someone on your direct paternal line. When you next go to your Paternal Line page you will see the option to set any share’s paternal haplogroup as your own.