Vsphere boot from CD rom

Hi, totally new to this and trying to work it out from the docos and various tutorials.
Want to run up a vm in vsphere that boots from an attached ISO on the cdrom.
Indications I can see seem say it should do that if the cd is configured with a bootable image, but when I do it it only tries to boot from the network interface.

I am not using a template or cloning an image, it is a fresh new vm.

Any guidance would be apprieciated.