Creates a custom field definition in KlickTipp and returns its ID and the placeholder that renders it in newsletter content. Creating a field stores no value and changes no contact. Fields are never created as a side effect elsewhere, so a field a contact should have a value in has to be created here first. The data type is picked once and for good: it cannot be changed afterwards, so pick the one that matches what will be stored -- a date field for a date, a number field for an amount. multiValue decides whether a contact holds one value PER SUBSCRIPTION (true, the default when omitted) or one value overall (false); it is worth deciding here, because an existing field can only go from true to false and never back. The name has to be unique within the account. The description is what tells a later reader what belongs in the field.