If you want to see this folder, you need a SSH connection to your DS system, maybe via PuTTY or WinSCP or similar, since any folder with @ at beginning is a hidden folder and is not visible in the DSM.
Therefore the folder /volume1/@appdata/syncthing is not a working folder for your data. In the DSM you need to give your Syncthing user/group the permissions of your folder you want to share and is visible in the DSM. Maybe you can also set in the web GUI this folder as standard.
I think you connect with your own account, but you need to connect as root. Best you visit the german Synology Forum to search for Threads, similar to the follow, to have a guide for this:
THX Forum was not very helpful - as always mixed up and messy
What worked for me at DSM 7.0.1-42218 was:
Log in With Putty as admin
sudo -i
sudo synouser -setpw root PASSWORD
cd..
vi /etc/ssh/sshd_config
remove # from line: PermitRootlogin yes
:wq
reboot
Start WinSCP
login as root with new PASSWORD
now I can find the folder:
/volume1/@appdata/syncthing
Not so easy. Is better to create with PuTTY generator a key, which is stored on the NAS and connect with WinSCP with key, user root and password of root. Thats it.