As soon as the reschedulement of the event has been announced by the event promoter or by Ticketmaster, you can request a voucher via the link in the email. You will find instructions about this in the same email. It will take up to 4 weeks before your voucher request is being processed. In case of a cancellation you will automatically receive a voucher, unless you request a refund.
You will in any case be informed proactively by email at the moment you can request a refund or voucher. From that moment you have 4 weeks to fulfil your request.