Hi,

You can override copy method of the sale order model and you need to call the super method. Copy method duplicates the record depend on the copy attribute which mentioned in the field definition, if the copy attribute is missing then the default value is false.

• copy (bool) – whether the field value should be copied when the record is duplicated (default: True for normal fields, False for one2many and computed fields, including property fields and related fields) 

In Odoo 14, there is an override for copy method on sale order to pass default value for validity_date depend on conditions:

https://github.com/odoo/odoo/blob/14.0/addons/sale_management/models/sale_order.py#L45

Update: 07-10-2021

The portal user will not have access to create sale order and you will do it using sudo(). If you able to get the sale order active record you will define a method and call it from the button and use copy method directly as below:
record.sudo().copy() and you can pass default values to copy method as dictionary

Here you are the copy method in documentation:
https://www.odoo.com/documentation/13.0/developer/reference/addons/orm.html#odoo.models.Model.copy