Chapter 3, pages 25 and 26 lists the parts and part numbers you need to add a hard drive...
(5) Hard drive bracket L20455-001
(6) Hard drive connector board L20454-001 (7) Hard drive/solid-state drive cable L20456-001
In my opinion, it would be a whole lot easier to just replace your existing 256 GB SSD with one having more storage capacity, and use the HP cloud recovery tool to make a bootable USB recovery drive to reinstall W10, the drivers and the software that originally came with your notebook on the larger capacity SSD.
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