You wish to setup a custom MAILHELO of an account on a dedicated IP address. You go to enable the "Reference /etc/mailhelo for custom outgoing SMTP HELO" option an find it is greyed out in the Exim Configuration Manager page.
How can you make the option available?
The option is being overridden by the "Use the reverse DNS entry for the mail HELO/EHLO if available" option. Disabling it will allow "Reference /etc/mailhelo for custom outgoing SMTP HELO" to become active.