To add a DMARC record on Plesk, follow the steps below:
1. Log in to your Plesk account
2. In Plesk, go to Sites Web & Domaines > Hébergement et DNS > DNS
3. Click on Ajouter un enregistrement.
4. Fill in the fields.
Type d'enregistrement: TXT
Nom de domaine: _dmarc
Enregistrement TXT: v=DMARC1; p=none
You can find more details in the article at the following link: What is DMARC?