Automatic forwarding sends a copy of incoming messages from your Yahoo Mail account to another email address. The forwarding address can't be one of your recovery email addresses. Upgrade to Yahoo Mail Pro or subscribe to Access + Forwarding to use the email forwarding feature in your Settings.
Click Settings.
Click Support Channels.
Click Mailboxes.
Click the primary mailbox account.
Under Forwarding, enter the Forwarding Address.
Click Verify.
Log in to your forwarding address and follow the instructions sent there.
Follow the official documentation here.