Easy way to add the Administrator user to the Welcome Screen:

Start the Registry Editor

Go to:

HKEY_LOCAL_MACHINE \ SOFTWARE \ Microsoft \ Windows NT \ CurrentVersion \ Winlogon \ SpecialAccounts \ UserList \

Right-click an empty space in the right pane and select New > DWORD Value Name the new value Administrator. Double-click this new value, and enter 1 as it's Value data.

Close the registry editor and restart.