Joe, we use this library for command line parsing. The documentation is more developer focused than end-user but you may be able to pick up some hints there.

My guess would be that you need to leave a space between your options and their parameters, and use double quotes around the filename. I haven't been able to test it, but try this:

TA.horizon.exe -i Acp -e TheSkyX -d "Custom Horizon.hrz"