code: def addfield(self, pkt, s, val):
          
          res = b""
          for p in val:
              res += self.i2m(pkt, p)
          if (isinstance(pkt, _GenericTLSSessionInheritance) and
              _tls_version_check(pkt.tls_session.tls_version, 0x0304) and
                  not isinstance(pkt, TLS13ServerHello)):
              return s + res
          if not pkt.type:
              pkt.type = 0
          hdr = struct.pack("!B", pkt.type) + s[1:5]
          return hdr + res
docs: Reconstruct the header because the TLS type may have been updated.
          Then, append the content.