I would like to add a 2.5 SSD to my A515-51-523X. It has an M2 and an empty bay for 2.5 but no hook-

Hi,
You need a SATA cable and the bracket, 4 screws to attach the HDD to bracket, 4screws for attaching the HDD assembly to the bay, if you find difficult to get the bracket and screws, you can use some non conductive material to secure the HDD in the bay.
Part numbers:
Cable: 50.GP4N2.004
Bracket: 33.GP4N2.002
M3.0x3.0 : 86.MXRN2.008
M2.0x3.0: 86.GCHN2.009