This is how to add yourself as an Installer to the child org you’ll be working on using Namespace.
This will allow you to use the same login and password from your original parent org user and allow you to see all of the ORGs you have added yourself to using this method in the ORG dropdown.
Starting from the Parent Org, go to Users > Users, open the filter on the right-hand side, toggle Namespace, and copy the namespace.
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