The easiest way to do this is to have a control on the form to enter the
invoice number.  Lets call it txtInvoiceNumber (clever naming, what?).  And
let's call the form frmInvLookup.

Then in the query builder, put the reference to the form and control in the
Criteria row of the Invoice Number column:

Forms!frmInvLookup!txtInvoiceNumber

Now all you need in the Click event of the command button is whatever
command you need to run the export.  I would also suggest you check to see
that a value is in the control and that it actuall exists in the table before
you do the export and get nothing:

   Do While True
       If IsNull(Me.txtInvoiceNumber) Then
           If MsgBox("No Invoice Number Entered", vbRetryCancel) = vbCancel
Then
               DoCmd.Close acForm, Me.Name, acSaveNo
               Exit Do
           Else
               Me.txtInvoiceNumber.SetFocus
           End If
       ElseIf IsNull(DLookup("[INVOICE_NUMBER]", "tblInvoice", _
               "[INVOICE_NUMBER] = '" & Me.txtInvoiceNUmber & "'") Then
           If MsgBox("Invoice Number " & Me.txtInvoiceNumber & " Not
Found", _
                   vbRetryCancel) = vbCancel Then
               DoCmd.Close acForm, Me.Name, acSaveNo
               Exit Do
           Else
               Me.txtInvoiceNumber.SetFocus
           End If
       Else
           DoCmd.TransferText........
           Exit Do
       End If