First of all, the CMS account is not a separate account with which you can log in through an oath. This is more like a privilege and is associated with one of your google / youtube accounts. This differs from the usual youtube channel management, where each channel has its own login credentials.

youtube account-selector-view, CMS name@email.com, , oauth CMS.
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, - 403 , . API YouTube https://developers.google.com/youtube/partner/.
: API , API, . :
Note: The YouTube Content ID API is intended for use by YouTube content partners and is not accessible to all developers or to all YouTube users. If you do not see the YouTube Content ID API as one of the services listed in the Google Developers Console, see www.youtube.com/partner to learn more about the YouTube Partner Program.
API, CMS, - ... 7-14 , API- Content ID. , , , .
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contentOwnerShips, . API- https://developers.google.com/youtube/partner/docs/v1/contentOwners/list#try-it param fetchMine = true https://www.googleapis.com/auth/youtubepartner-content-owner-readonly. :
{
"kind": "youtubePartner#contentOwnerList",
"items": [
{
"kind": "youtubePartner#contentOwner",
"id": "[CMS_ID]",
"displayName": "[DisplayName]",
"primaryNotificationEmails": [
"mail@random.xx"
],
"conflictNotificationEmail": "mail@random.xx",
"disputeNotificationEmails": [
"mail@random.xx"
],
"fingerprintReportNotificationEmails": [
"mail@random.xx"
]
}
]
}
CMS_ID, API onBehalfOfContentOwner.
, ,
"https://www.googleapis.com/youtube/v3/channels?part=contentDetails&managedByMe=true&onBehalfOfContentOwner=[CONTENTOWNER]&access_token=[ACCESS_TOKEN]"
"https://www.googleapis.com/auth/youtubepartner ".
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