App tracking: Can't capture ContextData key in Adobe Launch

I'm trying to create a product string via Adobe Launch by  JSON payload with the context data from the app (productCategory, productName, quantity, price, etc.)  from the app. But for some reason the values from these aren't passed along - it just ends up saying "Unspecified". I know I'm able to send the context data keys, because if I write {"contextdata": {"textstring":"This is a text string"}}, I can see that in Analytics. So the problem seems to be when I want to use data elements from launch, which are supposed to capture the value of the different context data keys. Does anyone how to refer to the context keys in Adobe Launch?