setValidationInOfficeJs.ts

public async setValidation(source: Excel.Range, target: Excel.Range) {
    Excel.run(function (context) {
      return context.sync().then(() => {
        target.dataValidation.rule = {
          list: {
            inCellDropDown: true,
            source: source.values.toString(),
          }
        };
        return context.sync();

      })
    });
  }