Yep, I found what I have missed, so this is a sample of draft working code:

Public TopLevel
TopLevel = CreateObject('oForm')
TopLevel.name = 'TopLevel'

DEFINE CLASS oToolBar AS ToolBar
	ShowWindow = 1	&& In Top-level form
ENDDEFINE

DEFINE CLASS oForm AS Form
	ShowWindow = 2	&& As Top-level form
	PROCEDURE Init
		THIS.Visible = .T.
		THIS.AddProperty('oToolbar', .NULL.)
		THIS.oToolbar = CREATEOBJECT('oToolBar')
		THIS.oToolbar.Show()
	ENDPROC
ENDDEFINE

Would you mind to add this as topic in the UT FAQ?

Regards