Commentaries about the services

How to create comments that customers use to write their opinion in mit app inventor ?
Please show the step how to do it

You might be able to use the CloudDB or FirebaseDB to adapt a chat app to your purposes. People who have the app would be able to post messages that everyone who has the app can read. Here are some possibilities;

https://appinventor.mit.edu/explore/ai2/clouddb-chat

Also see here for a simple method with firebase: