Am I missing something here? I want to let a user login with their username and password from my website using the API and show them their subscriber lists. However I cant see any way to get the client's id with just their username and password. Appreciate your help.
Both designer and client API keys can be obtained through the API. As the documentation specifies, this does require using their username and password in the HTTP basic authentication, but that will be handled appropriately by whichever wrapper you're using.
You can then use the client API key obtained in subsequent calls to the API and only the data relevant to that client will be returned. For example, getting your clients will, using the Client-specific API key, return only the clientID for that client, which you can then use to obtain their lists.