Basically how this works is that a separate xPub (extended public key) is needed for each website. This way each website's funds go to a separate wallet and you can easily identify the website you received payment from. Please follow the below procedure...
Step 1: Get a new xPub. How to get multiple xPub's from the same wallet?
Step 2: Set callbacks for each of the websites
You can go to "Merchants > Store > Add a new Store" and paste in your new xPub and click "Save changes".
Add a new store for each unique xPub key.
You can also add a "Tag" (a unique name) for each of your stores to help you identify the store.
Setting callbacks for different xpubs
Step 3: API Key
All the websites will use the same API Key. There is no need for a new API key. You can view your API Key on the Merchants Page.