CXone Studio Snippet INVALID_FUNCTION error with GetRESTProxy syntax

We are trying to call an internal API from a CXone Studio snippet. The documentation suggests using GetRESTProxy, but we keep hitting an INVALID_FUNCTION error at runtime. Our current syntax looks like this:

ASSIGN restProxy = GetRESTProxy("https://api.internal.com/data")

Is this function available in our environment, or is there a specific namespace requirement we’re missing? We’ve verified the endpoint is reachable via curl.