In theory, it's pretty simple - all you have to do is HTML encode the text body and add it as a parameter as in your example. There is even a method to help you do it:
HttpServerUtility.UrlEncode[
^]
However, it is probably a bad idea, and very likely to be a very, very bad idea. The problem is that the maximum length of a URL (including parameters) is not specified:
The HTTP protocol does not place any a priori limit on the length of a URI. Servers MUST be able to handle the URI of any resource they serve, and SHOULD be able to handle URIs of unbounded length if they provide GET-based forms that could generate such URIs. A server SHOULD return 414 (Request-URI Too Long) status if a URI is longer than the server can handle (see section 10.4.15).
Note: Servers ought to be cautious about depending on URI lengths above 255 bytes, because some older client or proxy implementations might not properly support these lengths. (HTTP specification:
http://www.w3.org/Protocols/rfc2616/rfc2616-sec3.html[
^])
The situation in the real world is more complex: it depends on the browser and other systems. For example, IE8's maximum URL length is 2083 chars, and it seems IE9 has a similar limit, but earlier versions had a limit of about 2000.
Dangerous! If at all possible, I would find another way to do this, especially if the content of the message you are going to add is outside your control.