How to add custom fields on the basic prestashop 1.7.8 contactform

[Diadh]

Hello,

I tried to add some new field on the basic contact form but it's seems that don't work.
I followed this tutorial https://www.gregorychartier.fr/fr/creer-formulaire-de-contact-prestashop/ (It's in french sorry).
When i receive the mail is write {telephone} and not the phone number i write in the contact form.
Someone know how to do that ? 

Thanks for your replies !