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That was a really old CEF1 example but most of the code can be reused in CEF3.
You will probably need to create a TCefPostDataElementOwn. Use a ICefPostDataElement interface variable to store the new TCefPostDataElementOwn and call ICefPostDataElement.SetToBytes or ICefPostDataElement.SetToFile with the binary data.
Then call TCefPostDataRef.AddElement to add the ICefPostDataElement.