To the OP - just read the small print of the page you linked in, your answer is there
"4GB Summer Data option is automatically activated for new voice prepaid customers after topping up in a single transaction, and can be activated free of charge by existing voice prepaid customers via m.vodafone.hu, and through 1270 between 5th June and 31st July 2017.
The 4GB of data becomes available immediately after activation and can be used for 30 days prior to using the data allowance included in the plan or orderd as an extra option, only once per prepaid account. The data can be used in Hungary and from EU countires."