Since youtube APIv3 revision on September 15, 2016, the Watch Later list id cannot be found in channels.contentDetails.relatedPlaylists.watchLater as it contains fixed string WL.
The revision states:
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The channel resource's contentDetails.relatedPlaylists.watchHistory and contentDetails.relatedPlaylists.watchLater properties now contain values of HL and WL, respectively, for all channels.
To be clear, these properties are only visible to an authorized user retrieving data about the user's own channel. The properties always contain the values HL and WL, even for an authorized user retrieving data about the user's own channel. Thus, the watch history and watch later playlist IDs cannot be retrieved via the API.
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The last part is pretty intriguing since I can't imagine how to obtain my WL id, hence I can't list the videos I've saved for later.
I checked also reddit and other SO questions (one, two, three, four) but without success. They usually suggest v2 or v3 before the mentioned revision.
The question: How can I obtain ids of my watch later videos?
