Why bot doesn't trigger the first message( welcome message) for the first time user deployed in MS Teams

Channels
1

Hello Team
the client’s bot is deployed in MS Teams,
when a new user opens the bot in MS Teams for the first time the bot doesn’t trigger the first message ( welcome message ).

what client’s ask is just like in web where the bot triggers the first message( welcome message ) after opening the bot similarly in MS Teams should happen.

can you please help me to understand WHY this is so ?
is there any approach we can trigger a proactive Welcome message ( or notification ) for first time user.

2

Same as Whatsapp, the intent needs to be triggered by an utterance.

3

yeah that is known.
wants to know WHY it is so.
similar to web, why it doesn’t trigger automatically for first time user when deployed in channel (MS Teams)