To send welcome message at the beginning you have to send proactive message.
A proactive message is any message sent by a bot that isn't in response to a request from a user.
This can include messages, such as:
Welcome messages
Notifications
Scheduled messages
You can refer below doc for sending proactive messages.
https://learn.microsoft.com/en-us/microsoftteams/platform/bots/how-to/conversations/send-proactive-messages?tabs=dotnet
Thanks,
Nivedipa
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