How to filter lookup field by site name

Question

I have a list like this:

Title Site
AAA Site1
BBB Site2
CCC Site2

which is referenced in a lookup column.

Requirements are to filter in the lookup column only the items whose "Site" property is equal to the current site - the site the user is currently in. And this is required in the New form.

For filtering, I'd rather go for the tecnique described here by @ronx.

But I have no clue for getting the current site, or a least the current URL to perform some strings lookups.