Note: I use RBW to access my Bitwarden password vault on the command line or in scripts, so I need to make sure it is unlocked before running Neomutt, hence the Exec line in the desktop entry.
RBW sudo pacman -S rbw
~/.local/share/applications/neomutt.desktop
[Desktop Entry]
Categories=Office;Network;Email;
Comment=Simple text-based Mail User Agent
Exec=xfce4-terminal -x bash -c '"rbw unblock" ; neomutt' %u
Icon=neomutt
Name=neomutt
MimeType=x-scheme-handler/mailto;
NoDisplay=false
Terminal=false
Type=Application
xdg-mime default neomutt.desktop x-scheme-handler/mailto