curl --request POST \
  --url https://payments.oaknetwork.org/api/v1/merchant/token/grant \
  --header 'Content-Type: application/json' \
  --data '
{
  "client_id": "app_abc123",
  "client_secret": "sk_live_xyz789",
  "grant_type": "client_credentials"
}
'

{
  "access_token": "eyJhbGciOiJIUzI1NiIs...",
  "token_type": "Bearer",
  "expires_in": 3600000
}