' script in the Body document
' ^^^^^^^^^^^^^^^^^^^^^^^^^^^
' add new variable to make the textbox reference visible outside of the Body document function
Dim textBoxToReceiveFocus
Set textBoxToReceiveFocus = Nothing
...
...
' here is your function in the Body document
Function ...
    ...
    ...
    Set textBoxToReceiveFocus = <I>referenceToTextbox1</I>
    window.setTimeout "textBoxToReceiveFocus.focus()", 100
    ' instead of just <I>referenceToTextbox1</I>.focus()
End Function