• Install Mendeley Desktop
    flatpak install flathub com.elsevier.MendeleyDesktop
    
  • Install LibreOffice
    flatpak install flathub org.libreoffice.LibreOffice
    
  • Open LibreOffice Extension Manager at Tools > Extension Manager or Ctrl + Alt + E

  • Locate Mendeley Desktop extension at /var/lib/flatpak/app/com.elsevier.MendeleyDesktop/current/active/files/extra/share/mendeleydesktop/openOfficePlugin/Mendeley-x.xx.x.oxt

  • Install it

  • Have fun :D

Reference: