A WeChat Official Account does not allow you to send promotional messages to your followers on a regular basis. However, WeChat does allow service accounts to broadcast 4 promotional messages to its followers per month.
NOTE: Conversation API supports 1:1 direct messaging to followers so broadcasting is not available.
With the Conversation API, it’s possible to send messages with promotional content during a reply to a message or when followers click on the custom chat menu or at the time when they start to follow Official Account.