We do not have a C# library, you would need to write your own. I can think of two possible approaches, based on which USB device driver you use.
You can use the VCP (Virtual COM Port) driver. With this method, Windows thinks that StimTracker is using a regular COM port (serial port). You can use standard C# function calls to open the port, send commands directly to StimTracker, and close the port when you are done. The list of commands that StimTracker accepts is fully documented. If all you want to do is send an event marker, see the section “DIGITAL OUTPUT COMMANDS — SINGLE PULSE”.
Write a C# wrapper around our own C++ library. You don’t have to wrap every single function; you can do it with only the functions that you are interested in (scan port, open, close, and send a pulse).
Our C++ library uses a different USB driver called D2xx. It provides better timing: 2ms delay with zero jitter vs. about ±5-6ms for the VCP driver.
If you end up developing C# code, and regardless of which method you use, we would appreciate you sharing it with other users. We can list C# support on this page and, of course, give the developers and the lab full credit for it.