You can easily setup multiple websites on your Smart Send account.
All you have to do is add each website to your user Create new team
Go to the menu in the top right corner, chose Create New Team and write your website URL in the Team Name
You can add multiple users, setup individual shipping agreements and sender address for each team
Once you have added the website you can find it in the menu. If you go to the website you will be asked to fill out the onboarding with a Sender Address and you carrier information. You will also get an API Token to use on your new website.