Usually, you’ll set the folder with your DOS games as root folder in Dosbox. In my case it’s this line (from expert settings, but you can also set it up in the GUI):
mount c: /storage/0000-0000/Games/PC
Which means, I have a folder called “Games” on my sd card (/storage/0000-0000), in which I keep several subfolders (C64, Amiga, SNES, PC, etc.). So I mount /Games/PC as drive C in Dosbox. From there, I launch the game like I would do on a real DOS PC:
cd synd
main (<- that's the game exe file, I avoid .bat files whenever I can).
No matter where you keep your game, the folder containing your "Synd" folder should be mounted as C:, do not mount "Synd" itself as C drive.
I hope this makes sense. 😉