Just type it in. make sure you include the 'protocol' definition at the
start:
http:// ...
ftp:// ...
mailto: ...
...
(Even gopher:// ... If you like)
It is often best to wrap the URL in angle braces as you see in my signature.
Note that the url will not be 'live' in the signature editing window or your
draft message window, but will become hot-clickable in most mainstream email
clients of any platform.