Go to the ANTS website and log in. (https://immatriculation.ants.gouv.fr/demarches-en-ligne/faire-une-autre-demande )
Instead of going to "Sell or give your vehicle",
select "Make another request"
It is mentioned:
"Cette demande concerne les motifs suivants : - Je n'arrive pas à déclarer ma cession dans le cadre de la téléprocédure : je vends ou je donne mon véhicule., etc...".
Fill in all the information requested, specifying that you want to declare the sale to a foreign company that does not have a SIRET.
Attach the various documents: crossed-out vehicle registration document, proof of address, CERFA, certificate of administrative status, proof of identity.
Once you have received payment for the car and you have deregistered it from your name, you should have an example of the document you see below. We ask you to send us this document to ensure that we can legally take the car to Denmark.