Web.config and storing credentials

Hi all
I'm looking for a way we can store username and password credentials in a web.config file & using these to re-direct to another site, once these are authenticated.
The process is as follows:
* User logs into our website with required credentials

* There is a link on our website which invokes a new webservice
* The webservice is a paid for subscribed service (which requires authentication)

* Usually, there is a web form for this type of web service but since we need to steam-line the process from initial application logon, the logon page is not required.
* So when the link on our website is clicked, the new webservice is invoked. Authentication should be done by using the credentials in the web.config file and user is redirected to webservice (avoiding logon page)
Could someone tell me using pseudocode or a small asp.net code example how we could achieve this process?
The key goal is to avoid user goi ...