The default starting map for all Team Fortress 2 servers is ctf_2fort. If you want to set it to something else then you need to add +map ph_farm_a5 to your startup command line for your server executable. You can obviously change ph_farm_a5 to whatever map you'd like to start with.
Alternately, if you can't modify your server's startup line, you can issue a map command in the autoexec.cfg It's recommended that you just fix it on the startup line, though.
I've setup a new PropHunt Redux Server Setup Steam Guide for installing PropHunt Redux servers, although it's still a work in progress.