Saaslet needs to be able to share cookies with your domain (please find more on this here). To do this, you need to proxy requests to https://api.saaslet.com through an endpoint on your domain.
This might sound a bit scary, but it's actually super simple. The most common way to do it is to set up a subdomain, e.g. saaslet-api.mydomain.com. For that, you need to add a CNAME record to your DNS provider's Zone file. This works a bit differently, depending where you host your app. Here are tutorials for the most common hosting provider:
Alternatively, you can also set up a proxy-forward or proxy-pass rule on your webserver. This way you can forward requests to https://yourdomain.com/saaslet to https://api.saaslet.com.
Here's how to do that with some of the most popular HTTP Servers:
Once you've set your CNAME or forwarding rule up, all that's left is to share your proxied URL with saaslet.js:
const saaslet = new Saaslet( 'YOUR_APP_KEY', 'YOUR_PROXIED_URL');