Вот как это сделать:
var departmentSheet = excel.Workbook.Worksheets.Add("department");
departmentSheet.Cells["A1"].Value = "Management";
departmentSheet.Cells["A2"].Value = "Administrator";
departmentSheet.Cells["A3"].Value = "Quality Engineering";
departmentSheet.Cells["A4"].Value = "Enablers";
var designationSheet = excel.Workbook.Worksheets.Add("designations");
designationSheet.Cells["A1"].Value = "Developer 1";
designationSheet.Cells["A2"].Value = "Developer 2";
designationSheet.Cells["A3"].Value = "Developer 3";
designationSheet.Cells["A4"].Value = "Developer 4";
// add a validation and set values
// the range of cells that will contain the validation
var validation = departmentSheet.DataValidations.AddListValidation("B1:B4");
// set validation rules as required
validation.ShowErrorMessage = true;
validation.ErrorStyle = ExcelDataValidationWarningStyle.warning;
validation.ErrorTitle = "An invalid value was entered";
validation.Error = "Select a value from the list";
// set the range that contains the validation list
validation.Formula.ExcelFormula = $"={ designationSheet.Name }!$A$1:$A$4";