You can post the information yourself if you have the DNA information that confirms the relationship. Wikitree doesn't collect or store any actual DNA data, so you need access to the actual DNA information for the person who is the DNA match to create the confirmation. If your grandfather didn't take a DNA test himself that you can match directly to, then you need to have a first cousin who is a match. If the match isn't here on Wikitree you can identify him or her by initials and relationship, e.g. "J.W., first cousin of Candy."
Here's what the Help page says:
- If your match is a first cousin on your father's side you can confirm your relationship to your father and his relationships to both his parents. If your match is a first cousin on your mother's side you can confirm your relationship to your mother and her relationships to both her parents.
Here's a link to the DNA Confirmation Help page: Help:DNA Confirmation (wikitree.com)
There's a really handy app (click on the Find menu, then Apps) the DNA Confirmation Citation Maker that makes writing the confirmation easy.