You can't use the Posting Key to reveal a higher key. You need to use your Master Password there. I would recommend that you store your higher keys in a safe place (offline) and only use them as needed.
You also can use the Steem KeyChain addon for your browser to securely access your Private Keys and for signing transactions without typing in your keys on a website.
I havent set some master password in my account. Where can I set my master password?
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In the registration process you received the Master Password for your account. All Private keys a derived from that password. If you don't know your Master Password or Private Owner Key, it is impossible to recover your account or change your current keys.
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