POST https://egg.5ch.net/test/bbs.cgi HTTP/1.0
Connection: close
Content-Type: application/x-www-form-urlencoded; charset=UTF-8
Content-Length: 283
Cookie: NAME=; MAIL=sage
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Referer: https://egg.5ch.net/software/
User-Agent: Monazilla/1.00 JaneStyle/5.00 Windows/10.0.19045
Host: egg.5ch.net

現状のPOSTだとこんな感じこれなら文字化けしなさそう