Multiple Selections on Google Spreadsheets

multiple_selections.gs

/** 
@OnlyCurrentDoc 
Ref: https://spreadsheetpoint.com/multiple-selection-drop-down-google-sheets/
*/
function onEdit(e) {
  var oldValue;
  var newValue;
  var ss=SpreadsheetApp.getActiveSpreadsheet();
  var activeCell = ss.getActiveCell();
  if(activeCell.getColumn() == 3 && activeCell.getRow() >= 4 && ss.getSheetName != "Categorias" ) {    
    newValue=e.value;
    oldValue=e.oldValue;

    if(!e.value) {
      activeCell.setValue("");
    } else {
        if (!e.oldValue) {
        activeCell.setValue(newValue);
      } else {
        activeCell.setValue(oldValue+'; '+newValue);
      }
   }
  }
}