Pass the parameter to the bootloader option in your ks file.
bootloader --append="crashkernel=128M@16M" --location=mbr
bootloader --append="crashkernel=128M" --location=mbr
Then enable the service and configure kdump.conf in %post to use the correct partition to write vmcore.
Have a look at Chapter 26. The kdump Crash Recovery Service
too as it might be helpful. Specifically, 26.2.3. Configuring kdump on the Command Line
should give you some guidance on what's required to configure it on a system from the command-line that you can then further incorporate into kickstart, as needed. How do I troubleshoot kernel crashes, hangs, or reboots with kdump on Red Hat Enterprise Linux?
is another good reference.