The steps are:

  1. Create the Cryptomatte material and assign it to an arnoldAov shader slot.
  2. Set the Cryptomatte material on an ArnoldGlobalSettings node, in Channel Definitions -> aov_shaders (add an entry and assign your Cryptomatte material).
  3. Create ArnoldOutputChannelDefine and RenderOutputDefine nodes for each of the Cryptomatte channels you care about: probably some of crypto_object, crypto_material and crypto_asset.
    1. Make sure to leave the type set to RGBA.
    2. Don't forget to set the channel on RenderOutputDefine to your channel created in ArnoldOutputChannelDefine.