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.