WeChat doesn’t have any opt-in requirements as it is not possible to send messages to WeChat users unless they follow your Official Account. When a WeChat user starts to follow your Official Account, you can reply to 3 messages within a minute.
You are not permitted to send more than 3 messages a minute, unless followers:
Initiate a chat with your Official Account, then you can reply 20 messages within 48 hours with every reply.
or they click on the custom chat menu on the bottom, then you can reply 3 messages within a minute.