Assuming the operating system already exists, go to “Hosts > Provision Templates”, select the matching one like “Kickstart default PXEGrub2”, on the “Association” tab move the operating sytem from applicable to selected and submit.
Afterwards go to “Hosts > Operating systems”, select the operating system, on the “Templates” tab make the associated template the default for Grub2.
Now in the Host wizard you can chose the bootloader.