You can provide users to a with your own branding, but first you will need to create the main user to have access to this portal.
To create this user, navigate to the Edit Customer screen: Customer > List All > View > Portal Login Information.
Edit Customer screen with the Portal Login Information tab open, showing Username and Password fiel…
Once you have set a Username and Password for the user, they will now be able to access the white label portal using the credentials you have setup for them.