[SalesForce] External CSS-Files for Microsites

It's really no different than any other platform used for making API calls. How would you build your API call to fetch subscriber preferences in a LAMP or IIS environment?

If the API action needs to be on a SFMC Landing Page or Microsite, then you'll have to do it within the confines of SFMC's SSJS and AMPScript functions.

If you need to do an HTTP Get on a public endpoint, then you can use the SSJS Get() or the AMPScript httpGet() functions.

Email object does have a CustomerKey attribute, as far as I know it is not visible on Email Studio page.

The only way for me to find the CustomerKey for a classic email is to make SOAP API call, RetrieveRequest for Email object, make sure "CustomerKey" in the return properties list.

This is the guide for Retrieve email via SOAP API