If your user is a database owner, (i.e. is a member of the db_owner group or has CONTROL permissions on the database) the default schema will always be dbo. This is something you can’t change.
So if your legacy application needs quasi-administrative privileges in the database, you can’t make it a database owner, but you can grant those permissions on the schema instead (which is actually a better idea anyway).
What Daniel is doing is akin to the pre-2005 concept of user spaces, where Bob had a schema and Mary had a schema and Jill had a schema and so forth.