How can I put an IP on the allow relay host on exim via cpanel?

To allow relaying from a particular IP in exim, you would log
into WHM, click on “Exim Configuration Editor”, and then click on
“Switch to Advanced Mode”. If you wished to allow relaying from the IP
of 1.2.3.4, in the first text box, you would put the following:

hostlist relay_hosts = lsearch;/etc/relayhosts : \
    localhost : 1.2.3.4

The scroll down to the bottom and click on “Save”.

Cpanel