Under Home Screen search for storage accounts or select it from the azure services bar.
Select the storage account that was created from the "creating a storage account doc" by searching or finding it in the list.
Once you have selected the storage account on the left navigation panel under Security + networking click "Access Keys".
In the access keys tab at the top click "show keys".
Under key1 the value listed under "key" is your storage_key that will be used as the input for the ingest portion of the config.json file.