Canvas does not have a feature that notifies your student when you're online. The only way to accomplish this that I am aware of is to tell them in the syllabus that you will be online during these times each day or week or you can send them a conversation each time you are on and if they have their notifications setup to receive a message immediately upon getting a new conversation they could get alerted that way as well.
Hello, Ciro, Brian has given you great advice about communicating online times through the syllabus and via messages in the Inbox. If your institution has enabled Chat, your students will be able to see who is online by navigating to the Chat tool, although no notifications are sent when chat members join or leave the chat. Please refer to How do I use Chat as an instructor?
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