insert a link in to a email send using c#

You should be able to just add the mark-up for the link in your body variable:

body = "blah blah <a href='http://www.example.com'>blah</a>";

You shouldn't have to do anything special since you're specifying your body contains HTML (m.IsBodyHtml = true).

 String body = "Your message : <a href='http://www.example.com'></a>"
 m.Body = body;

Within the body. This will require that the body be constructed as HTML so the that a or can be used to render your link. You can use something like StringTemplate to generate the html including your link.