The option to add a digital signature is not available in QuickBooks as of now. You can submit your feedback directly via the Feedback widget in the product. That way, your feedback directly reaches the Product Development Team and helps improve your experience faster.
I am not sure it wil be working on QBO India version, but you may try it as workaround. Utilize the starter plan of this connector for free. Your invoice will be sent to GoogleSheet and signed by Eversign/ZohoSign.