正确的发送客服消息API是如下链接：

https://api.weixin.qq.com/cgi-bin/message/custom/send?access_token=ACCESS_TOKEN

正确的文档链接为：https://developers.weixin.qq.com/doc/offiaccount/Message_Management/Service_Center_messages.html#7

而文档链接 https://developers.weixin.qq.com/miniprogram/dev/OpenApiDoc/kf-mgnt/kf-message/sendCustomMessage.html 里的链接https://api.weixin.qq.com/cgi-bin/message/custom/business/send?access_token=ACCESS_TOKEN 是错误的，多了一个“business” 目录。