CXone Studio GetRESTProxy returning INVALID_FUNCTION error

Trying to call an external API from a CXone Studio snippet. The GetRESTProxy() function keeps failing with an INVALID_FUNCTION error. I’ve checked the docs, but the syntax is vague. Here’s the code:

ASSIGN rest = GetRESTProxy('https://example.com/api');

Is the syntax wrong, or is it a permission issue on the file? The endpoint is public, so auth shouldn’t matter yet. Tried adding headers but same result. Any ideas?