Kindly find the Dell Technologies PowerEdge R240 Installation and Service Manual on https://dl.dell.com/content/manual60568834-dell-emc-poweredge-r240-installation-and-service-manual.pdf
See the diagram below for backplane cable routing (the cable indicated in red is the backplane power cable).
Backplane connectors on the diagram below:
The R240 Server's cable power supply unit (PSU) close look is shown below.