Welcome to APG, to answer your qestion all of the XenForo forums work with something called "BBCode" which you can access easily here:
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Spoilers etc. are here:
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There is also a preview feature to see how your thread looks like on the right side
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You can also use AI to format your therads, you'd have to ask it for the BBCode and then press here:
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And insert the code, and then press the code thing again to leave the mode again because it fucks up the post button