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How can I do this? I'll show you an image as an example:

"Pre content"





I already searched a lot but found nothing about it

Something like this requires a custom plugin. As it's an odd feature not many will want you may not get it for free. The plugin will have to hook into newthread_start, create two new inputes and paste those inputes inside the general message. Not too hard at all for someone who knows what they're doing.