Which field is to be populated for Contact addition for PAN number?

Which field is to be populated for Contact addition for PAN number?

We are using zoho books api to create contact details directly from our website.  We are getting 'Invalid value passed for tax_id' or 'Invalid value passed for tax_reg_no' error when 10 digit PAN Number is passed.

How to populate PAN number?

We are able to add a contact if 15 digit GST number is passed to tax_reg_no.  However, the issue is encountered only with PAN number.

Please suggest.

Thanks