вся инфа тут, можно взять даже client_id и client_secret:
https://developer.yahoo.com/oauth2/guide/openid_connect/getting_started.html#getting-started-setup
твоя ошибка в том, что ты не указал валидный scope.
scope=openid%20mail-r доступ к чтению почты, а твой client_id не разрешает запрашивать доступ к чтению оной.
Порядок действий:
1. Получаем код авторизации:
https://api.login.yahoo.com/oauth2/request_auth?client_id=dj0yJmk9aVhnUWtwTDRnZnJ6JmQ9WVdKalpHVm1hQS0tJnM9Y29uc3VtZXJzZWNyZXQmeD05Mg--&response_type=code&redirect_uri=oob&scope=openid%20mail-r&nonce=YihsFwGKgt3KJUh6tPs2
2. Запрашиваем токены:
POST https://api.login.yahoo.com/oauth2/get_token HTTP/1.1
User-Agent: Fiddler
Content-Type: application/x-www-form-urlencoded
Host: api.login.yahoo.com
Content-Length: 201
grant_type=authorization_code&client_id=dj0yJmk9aVhnUWtwTDRnZnJ6JmQ9WVdKalpHVm1hQS0tJnM9Y29uc3VtZXJzZWNyZXQmeD05Mg--&client_secret=09ed456f928682009b5132372a0beede39c82fa3&redirect_uri=oob&code=wzwqa4z
//
{"access_token":"IOWke9yYpQrtCekK2BUn.aePljQdMjACuz1GJPXyYM2VkhUlmnH_OSr5JMBDUKj6IVMGS6CLNxlXUeES2Ix6mMGNNRgoAwWvApM0CyVV4mxuHWR0xmQFros3iEgII2PKPaR70eY7f.supKDn9ea5pHKCPZvo0Mafs0EIGtdePFsWi0uXiBJJLcfVG6UDNY8emFO8yec249mooB2TYIneEujybKhtggAL6ttxvVGsDy5jik05MmrqAfugXZZRyY6_KzIwZ24cIGFCw4rm7HMm8uM7RjT1C3Rv0aGjpAIrOTHSus4pWf78c57sSJwzMlJtFXEdl.4Yyo9gkSxjWyqbsiShtAedmdwdjXmTIGMNwWcPYUungGvkTyG9yw70TBf4.jZSGbhmRcMrxPQUmAIEARMUWYfUqdEjKAP7TTr.VnLUV3ZqJlWNClaLNceSMD276DjldwltOPbZfq7HYhR5J8xP1Bcp.E0XDkpzZo8BmG1gPaGnCxeUSrWg.o5JqQzhkMvUvYKv_QpXs6UIuu7Pq53tfP.8AuhMN0u89c8ggw2oT9llObrRbRXA62YeSpgnrWDtUONn01dkWnFhRf4su7nosLDn85P60s1RHQAMyRJI3_4TIm_aCHowujx3myKOFVa94GHsIqXZdh5v5jR6bGRME_vwv5HoWRldguXFPWsuQs.pQjK0FtwUWr4nJVW94QnVNrafOn82YyvMQqcNJSvhiiQWAwmRfEhDMEkbD5BCCDQdbQ_B24GiqYomrhEE9jPIbUynbZQoJZzeclJ6Ubg9N1T7_imbMIIeWoQpmeQbzUem9AskrN6y3Gux_zKlQm0uuWzaARtSPvq_Jj5fO0j0dscHj1sSzDPyuo42MsnH4tw1OYxgMgfPjrbRnNovgghMv09grynBMQQ2HMmDvvfR4Jikfsarn52YBV7r.I5sEPMgYCkRprjFGOU-","refresh_token":"AEgYdGiG1maEWaocK4Oi5oRzjCsl~001~t0ZxeuXOIN2CUsgRPR4s1OHRUEs-","expires_in":3600,"token_type":"bearer","xoauth_yahoo_guid":"EU63NPRIHTBRAZ6GRDMFL6MMW4","id_token":"eyJhbGciOiJFUzI1NiIsImtpZCI6IjM0NjZkNTFmN2RkMGM3ODA1NjU2ODhjMTgzOTIxODE2YzQ1ODg5YWQifQ.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.NggLzsWVmGzaq_vCBYcIcNLZ6T3H6yu05GcgjUR9Q1SVEgMh-H6P8wHky65_7EV4qyLmSegUduD2cb1FHznh7A"}
Вуаля, все работает: