This can be done via the Business Dashboard using the Menu Editor under Online Ordering
Follow the steps given below:
1. Select Menu Editor under Online Ordering on the dashboard navigation list.
2. Find the Item you want to add a tax for by picking them from their relevant Category / Sub Category and select the same.
3. On selecting the Item, you will see the Item Details tab for the same, scroll down to Advanced Settings.
4. From the GST dropdown list, select the relevant rate of tax.
5. Once you've made the edits click Save.