What you've got there is VBA code, not a macro.

I gather you're trying to type that expression into some event of your text
box. Rather than typing the expression there, select [Event Procedure] from
the pull-down list for the event property, then click on the ellipsis (...)
to the right of the box. The VB Editor should open up, with something like:

Private Sub TextboxName_EventName()

End Sub

Type your expression between those two lines.

(Note that depending on what expression you're using, there may be
parameters in the parentheses.)