Hi.
For now it is only in 2.79 from Master, it is not in 2.8 branch. You have to compile 2.79 from Master by yourself or wait for tonight for buildbot builds.
You have available settings in Render Layers tab under Passes item.
Then in compositing nodes you find the node under Matte group.
What I do is render. Then in compositing nodes I choose Cryptomatte “Pick” output. Then I choose objects or materials with Cryptomatte “Add” selecting pipette from UV/Image editor.
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