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As you said, there is no such restriction on posting questions in the forum section or replying to any question of the forum section. You can reply or answer to any thread in the forum section by typing your words in the Post Reply box and then click on Submit Response button.
If you are talking about Ask Expert section, and referring the Ask Expert section as Forum Section, then it is a different case. In Ask Expert section, you may not get access to post answers to the questions immediately after joining the site.
With Best Regards,