How to apply TDS on a proforma invoice?

Sorry, it is not possible to apply TDS on pro forma invoices.
You may apply TDS on actual sales invoice after converting it from pro forma invoice.

To convert pro forma invoice into sales invoice please follow the below steps:

Go to Quotations>> Click on Pro Forma Invoices<< Select desired pro forma invoice that you want to convert and click on three dots shown<< Click on Convert<< Convert to sales.

And to apply TDS to sales invoice please refer to below linked article: