Every line in /etc/fstab becomes a .mount unit at boot time, created by systemd-fstab-generator in /run/systemd/generator/. Because it’s generated, systemd refuses to touch it:

$ sudo systemctl add-wants media-external.mount myapp.service
Failed to add dependency: Unit /run/systemd/generator/media-external.mount is
transient or generated.

The way out is a drop-in in /etc/systemd/system/, which is merged even on top of a generated unit:

sudo mkdir -p /etc/systemd/system/media-external.mount.d
printf '[Unit]\nWants=myapp.service\n' | \
  sudo tee /etc/systemd/system/media-external.mount.d/10-app.conf
sudo systemctl daemon-reload

Corollary: deleting the line from /etc/fstab doesn’t remove “just the automount” — it removes the whole unit, and any BindsTo= pointing at it breaks.