It is intentionally protected. Auto posting is a method used by spammers, so forum software uses prevention methods. Even if you manage to get it to work, the second it is detected, it is likely to trigger a response that will block the IP and possibly ban the userid.
When you want to add such an ability on any site, you start by asking the operator/owner of the site if you are allowed to do such things, and then they may even help you with it.
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