Ensuring textbox has specified number of digits

Perhab like this
Or you can do using textbox_upterUpdate

private sub textbox1_change()
Dim t$,t1$
If isnumeric(textbox1.value) = true then 
  t = textbox1.value
  t1 = 000000
  If len(t1) > len(t) then
    Textbox1.value = StrReverse(Replace(strReverse(t1),0,t,1,len(t)))
  End if
End if
End sub