如何使用netsuite suitescript 2.0版本为多选字段设置值？

I want to set the value for the multi-select with existing values in that field. (i.e) If the Filed has the values "A , B" means I wants add the New Value "c" with Existing Values So, Result would be "A,B,C" . I want to set the value for the multi-select wi