Your web service user credentials for Affilinet can be found in the Affilinet UI.
If you click the following 

Then scroll to the bottom, you should see Web service account management.
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Under that you can create a new user or if you have an existing one you can use that. The password you need is the one for the web service user and not the account. You can get one with the button named "create new password" for the specific user.

Make sure the user you use has access to the program ID you are connecting.