In subscription Sub1 you created a custom Event Grid topic Topic1 and an event subscription EventSub1 that uses Topic1 as the source and a Web Hook as the endpoint. You plan to enable dead-lettering for EventSub1. What must you do first to enable dead-lettering?

To enable dead-lettering for an Azure Event Grid event subscription, you must first specify a storage account and a container within that account where undeliverable events will be sent. Event Grid uses Azure Blob Storage for dead-lettering. Therefore, creating an Azure Blob Storage container in the same subscription (Sub1) is the necessary first step. Configuring delivery properties or the retry policy of EventSub1 are subsequent steps or related configurations, but they do not establish the required storage destination for dead-lettered events. Creating an Azure Storage queue is incorrect because Event Grid dead-lettering specifically uses Blob Storage, not queues.