To get a new line should use ASCII value like %0D%0A
Below is my updated working code...
string toEmail = " ";
string body = string.Format("I would like to share a link from the Nasdaq Reference Library, click the link below to view search results.%0D%0A{0}", MailUrl);
string subject = "Nasdaq Listing Center Reference Library Link";
string message = string.Format("mailto:{0}?subject={1}&body={2}", toEmail, subject, body);
Process.Start(message);