curl --request GET \
  --url https://perp-api.phoenix.trade/v1/market/{symbol}/mark-price \
  --header 'Authorization: Bearer <token>'

{
  "slot": 1,
  "slotIndex": 1,
  "symbol": "<string>",
  "markPrice": {
    "price": 123,
    "slot": 1
  }
}