let dashboard;
let worksheet;
if (viz.workbook.activeSheet.sheetType === SheetType.Dashboard) {
  dashboard = viz.workbook.activeSheet;

  // Provide the name of the worksheet you want to use from the dashboard
  worksheet = dashboard.worksheets.find((ws) => ws.name === 'Replace-Name-of-Worksheet');
} else {
  // Active sheet is already a worksheet
  worksheet = viz.workbook.activeSheet;
}