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It sounds like you've got some progress with pro-active messaging already. Is it working 100%? If not, I've covered the topic a few times here on stack overflow - here's an example that might help: Programmatically sending a message to a bot in Microsoft Teams

However, with regards -trigging- the pro-active message, the truth is you can do it from anywhere/in any way. For instance, I have Azure Functions that run on their own schedules, and pro-active send messages as if they're from the bot, even though the code isn't running inside the bot at all. You haven't fully described where the Angular app fits into the picture (like who's using it for what), but as an example in your scenario, you could create another endpoint inside your bot controller, and do the work inside there directly (e.g. add something like below:)