I can get this to work if I have a steam_appid.txt file with the steamworks SpaceWars example appID of 480 and use uint32 k_unSecretData = 0x5444;

However, it returns k_EResultFail if I utilize the app's proper appID with k_unSecretData = 0x5444.

The SDK tells me that k_unSecretData is "app specific shared key".

But where to get the value from?

Can anyone help me?

The Steamworks partner site should allow you to generate a new key for a given application / app ID, assuming you have access.