First off, your HK416 looks like it has an LMT Crane stock instead of an HK416 stock.
As mentioned, either a receiver plate with some kind of sling attachment point, similar to:
or if you want to use QD sling swivels:
There's also an adapter you can install on the Crane stock for QD swivels:
Or get the newer Mk18 Mod 1 version of the LMT Crane, it has a QD swivel stud built-in (be sure to get the right one if you're using it to store an AEG battery):
Or you can also oldschool/low-tech it and run a loop of paracord through the slots and attach your sling to that.
edit: you might think that last one sounds Ghetto As Fuck, but it's the quietest setup you'll find.