[SOLVED] invoice & order

Using the customer invoicing functionality, I want to display on the invoice the external reference provided by the customer (the field [command.externalReference]).
This field always remaining empty in the generated invoice, I do not understand how the link between an order is made is an invoice?

I tried to put the order as the origin of the invoice, without success.
Thank you in advance.

Thank you.
I had looked in the available fields, and indeed the field[command.externalReference] is not present, however, I wondered if it was not an omission in the documentation because this field appears in a template delivered with the plug in (temp_bill_en)...
it doesn't matter, I'll add the external reference to the generated invoice, but this feature seemed convenient to me.