Activate the debug mode and go to Settings > Technical > Database Structure > Fields

Create a new field and give it a name. Since we want to add it on an invoice, the model is “Journal Entry”.

If your invoice was created from a sales order, each invoice lines are linked to a sale order line.

So we need to create the related by field by retracing the way from the invoice line to the field you want on the sales order.

The expression you need is: invoice_line_ids.sale_line_ids.order_id

This field will give you the sales order related to the invoice. If you want a specific field from the sales order, for example the order date then I juste need to add .date_order at the end of the expression.

So: invoice_line_ids.sale_line_ids.order_id.date_order

Now that the field is created, you can add it on the view with Studio.

Be careful that it will only work correctly if the invoice was created from only one sales order.

If you have several sales order related to the invoice, the related field will take the sales order linked to the first invoice line.