Okay, I finally managed the sql syntax:

DoCmd.RunSQL "INSERT INTO PlanningChangeLog(" & _
        "ID, TimeStampEdit, UserAccount, Datum, Bestelbon, Transporteur, Productnaam, Tank) " & _
        "SELECT ID, Now() as TimeStampEdit, '" & user & "' as UserAccount, Datum, " & _
        "Bestelbon, Transporteur, Productnaam, Tank FROM Planning " & _
        "WHERE Bestelbon = " & Me.txtSearch & ""

This copies the record to a changelog table, and inserts a timestamp and user account field after the index field.

Thx for all the suggestions!