Unable to send pro-active messages to users on Microsoft Teams

May consider this:

How to Fix It
1. Publish Your App to the Teams App Store
- Once certified, Microsoft grants your bot multi-tenant access.
- External users can install it and receive proactive messages.
- Follow Microsoft’s publishing guide to submit your app.
2. Use Federated Identity Credentials (FIC)
- Add an FIC to your bot’s Azure identity.
- This allows token exchange across tenants using MSAL or Bot Framework REST API.
3. Handle Tenant-Specific Authentication
- Use https://login.microsoftonline.com/{TENANT-ID}/oauth2/v2.0/token for token acquisition.
- Avoid using /common or /botframework.com endpoints—they no longer work for new bots.
4. Ensure Bot Is Installed by the User
- Proactive messaging only works if the bot is installed in the user’s Teams client.
- Use the conversationReference from the installation event to send messages.