The ID and password is exactly the same as for Yahoo websites. If you forgot it then it can be changed or retrieved the same way.
If you forget your Yahoo IDs you can get a list if you used the same alternate email address for all of them.
If you forget your Yahoo password or someone has changed it you must provide your date of birth and original zip code. A new password will be sent to your alternate email address. If there was no alternate email address then the secret question will be asked. You must follow those steps. There is no way around it.
Here is a form for forgotten IDs and passwords.
US: https://edit.yahoo.com/forgot
UK: https://edit.europe.yahoo.com/forgot
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