updateMultiSelectPicklist.html

 SET @preferences = CONCAT(
      Iif(RequestParameter("newsletters") == "on", "Newsletters;", ""),
      Iif(RequestParameter("events") == "on", "Events;", ""),
      Iif(RequestParameter("offers") == "on", "Special Offers;", ""),
      Iif(RequestParameter("surveys") == "on", "Surveys;", ""),
      Iif(RequestParameter("promotions") == "on", "Promotions", ""),
    )  

 if not Empty(@preferences) then
  SET @updateRecord = UpdateSingleSalesforceObject(
      "Contact", RequestParameter("subkey"),
      "FirstName", RequestParameter("firstname"),
      "LastName", RequestParameter("lastname"),
      "Email", RequestParameter("email"),
      "Preferences__c", @preferences)
  else
    SET @updateRecord = UpdateSingleSalesforceObject(
      "Contact", RequestParameter("subkey"),
      "FirstName", RequestParameter("firstname"),
      "LastName", RequestParameter("lastname"),
      "Email", RequestParameter("email"),
      "fieldsToNull", "Preferences__c")

  endif