Adding answer from comment section for more visibility:
@Serban - You can send proactive messages while installing/adding bot in Teams channel.
Reference Docs:
https://learn.microsoft.com/en-us/microsoftteams/platform/bots/how-to/conversations/send-proactive-messages?tabs=dotnet
You can also use Graph API to send message in channel based on your requirement:
https://learn.microsoft.com/en-us/graph/api/chatmessage-post?view=graph-rest-1.0&tabs=http
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