I am trying to use Salesforce DX so I can create scratch orgs for my managed package, but I need to create a scratch org w/ the same namespace. I found this article https://developer.salesforce.com/docs/atlas.en-us.packagingGuide.meta/packagingGuide/sfdx_dev_reg_namespace.htm but I do not have an app called "Namespace Registries" (step 3). 

How do I get that app or how do I create a scratch org with the same namespace prefix as my managed package?
Thanks