Centre d'aide / Votre compte
Open the account menu (person icon) → Account and security → Set up two-step sign-in. Add the key to Google Authenticator, Microsoft Authenticator or Authy and type the 6-digit code it shows. From then on, signing in needs your password and a code from your phone.
Lost your phone? Contact support; after confirming who you are, an administrator can turn it off so you can set it up again.