Dell requires the serial number for the drive enclosure when replacing a failed drive.  This is a two step process that must be run as root via the CLI.

You will need the adapter number to determine the serial number of the enclosures attached.

Running lsi-show will show the Enclosure Information with the Id.  This is known as the Adapter number when running megacli.  Then running megacli will show the serial number for the Adapter.

For example, running lsi-show, the Adapter number is a2 in this case:

Controller Information
Id   Serial No        Firmware             Encl LD   PD   Product Name
a2   24P005M          3.131.05-3415        1    1    12   PERC H810 Adapter

Enclosure Information
Id   Drives
25   22  21  13  16  15  14  19  18  17  20  23  24

Running megacli -encinfo -a2 | grep Vendor will show that DELL is the vendor name, and the serial number.

In the example below, the serial number is HSH5T2.

 megacli -encinfo -a2| grep Vendor
        Vendor Identification     : DELL    
        Vendor Specific           : HSH5T2

Remember, when requesting a drive replacement for a failed drive, include the line from the lsi-show output indicating the failed drive, the enclosure serial number, along with the shipping address and contact name and phone number.