Submit solution for current candidate

Example request

{
  "pk": "0350e25cee8562697d55275c96bb01b34228f9bd68fd9933f2a25ff195526864f5",
  "w": "0366ea253123dfdb8d6d9ca2cb9ea98629e8f34015b1e4ba942b1d88badfcc6a12",
  "n": "0000000000000000",
  "d": 987654321
}