In D365 F&SCM, for a customer or vendor, in the Invoice and delivery section, you can enter a tax exempt number. Before you can do so, you must manually set up the tax exempt number.
In Data Entry Workflow, you can setup a Data Entry Workflow template to create a customer or a vendor. For a data entry step in this template, you can select the Tax exempt number field.
When a Data Entry Workflow is started, on data entry, you can enter any tax exempt number.
On transfer to target, if the entered tax exempt number:
Exists, this number is used.
Does not exist, it is automatically created.