Secured Web Servce and Business Office Programming

[C# Programming]

Hello,

I am coding an AddIn for Word and Excel in VS .Net 2008 in C#.
In this AddIn I want to use a secured web service.
I suppose that I need to have login and password that I'll pass in my web method.
The URL begins with "https://..."

The principle :
- I load a Word document from an authenticated website (same site that the web services)
- I have a ****** that contains my sessionID in my browser

My question :
Where can I stock the login/password I have to use for my webmethod?
Do I read these informations from the ****** of my browser (directly from my Word AddIn?)
How else?

Thanks in advance.
Yannis.