試しもしないで適当に書くと
Content-Lengthはcurlに任せて
request.headers["Referer"] = "https://"; .. host .. "/" .. board .. "/"
他にAcceptとAccept-Encodingを合わせて
request.body = createBodyFromTableWithOrder(tBody, {"submit", FROM", "mail", "MESSAGE", "bbs", "key", "time"})