According to this the docs, one should be able to use setScriptData in the Request handler and retrieve that with getScriptData in the Response handler. I'm finding that I only get null in the Response handler. Specifically, I'm looking at DeviceAuthenticationRequest and AuthenticationResponse.
I tried to test with a fresh Request+Response pair: AccountDetailsRequest. Appears that AccountDetailsResponse is called, but AccountDetailsRequest is never triggered (in the Test Harness with Debug Requests checked, it doesn't debug).
Tested with ListAchievements*. Both scripts get triggered at least, but still experiencing the same behaviour: getScriptData returns null.
Okay, I figured it out. You set it in the request with setUserData(key, value) and get it with getData().key. https://support.gamesparks.net/support/discussions/topics/1000055250
VERY confusing and underdocumented.
We're about to begin a major overhaul to our documentation Nick.
I've added a note about what you mentioned re the scriptdata specifically.