Can I run rackup and point to a specific config.ru?

I’m trying to setup sinatra as a service on Windows. I’m using the sc
command to create a service, but I’m stuck on the binary path.

I would like to do something like:
'rackup -p 80 c:\mysites\site1\config.ru"

But I can’t seem to find any rackup switches to do that. From the
command prompt with Ruby in the path it’s easy to accomplish manually,
but it doesn’t work when the service tries to run rackup from within the
site where rackup.bat doesn’t exist.

Is there a way to specify a config.ru?