How to add values in multi select lookup field in sharepoint using CSOM

Create a List of FieldLookupValue's
List<FieldLookupValue> oLookupValues = new List<FieldLookupValue>();

Add a new FieldLookupValue for each file id
oLookupValues.Add(new FieldLookupValue() { LookupId = Convert.ToInt32(lookupID) });

When finished set the listitem to the List
listItem["FieldName"] = oLookupValues ;
listItem.Update();