SRX

How do I configure an IP helper to direct traffic to a PXE boot server please?

The helper configuration is under forwarding-options helpers. See https://www.juniper.net/documentation/us/en/software/junos/cli-reference/topics/ref/statement/helpers-edit-forwarding-options.html

The Juniper documentation for this feature is slim to misleading; you're probably better off just using the "?" in the CLI.

Depending on what solution you're deploying, you may be able to just specify DHCP options and not need IP helpers at all. But for other solutions (e.g. Microsoft) you'll need to configure a TFTP helper, and maybe also a DNS helper, not sure.