curl --request GET \
  --url https://api.prime.coinbase.com/v1/entities/{entity_id}/futures/balance_summary

{
  "portfolio_id": "00000000-0000-0000-0000-000000000000",
  "cfm_usd_balance": "1000.00",
  "unrealized_pnl": "10.12",
  "daily_realized_pnl": "10.12",
  "excess_liquidity": "10.12",
  "futures_buying_power": "10.12",
  "initial_margin": "10.12",
  "maintenance_margin": "10.12",
  "clearing_account_id": "CFS1234567",
  "cfm_unsettled_accrued_funding_pnl": "10.12"
}