a JSON object where a key is account # and the value is the number of subscriptions for a custom message
Publish custom messages to subscribers
POST /custom-message
Parameters:
accounts (in body) is a list of accounts that are targeted for a custom message (should be subscribed within stream message type CustomMessageSubscribe)
message_body (in body) is a body of a custom message (must be a valid JSON object)
response
a JSON object with a flag of publishing the message