Link subpage on Nav Menu to outside url

I have “Contact” on my menu with a subpage of “Goodreads”. When a person hits the “Goodreads” subpage button I want them to be automatically transported to my Goodreads profile page. I have the link url from Goodreads but I don’t know where to put it and the total format for doing so.
My website is http://www.darkhavenbookreviews.com
Since I want to do this for facebook and twitter as well I REALLY would appreciate some assistance.

Thanks much.