Send a message to all users (like a broadcast)

Thomas_Steibl Another option is InformaCast Fusion. You can find the app here:

https://appsource.microsoft.com/en-us/product/office/WA200000080?tab=Overview

InformaCast has an integration with Microsoft Teams and is built for both small and enterprise deployments. The best part is it can be deployed very quickly with little effort. Here's a link to an overview: https://www.singlewire.com/blog/emergency-notification/add-microsoft-teams

You'll need licensing so it does have a small cost associated with it. That said, it's the easiest and most scalable way of sending the broadcast to all Teams Users. The cool part here is when you license the Teams User you also license the user's desk phone, PC, and mobile device. So you can reach all of your users across all those devices simultaneously if you wanted. Additional functionality is available that allows for things like full-screen takeover on the PC, intrusive audio via their desk phone, SMS, Push, and more. You can of course trigger these notifications right from Microsoft Teams. It's a really cool app. I recommend checking it out. 

Thomas_Steibl  Yes it's possible, You may create an Incoming webhook available in the App store. The Webhook will give a unique URL to you. You can post the message using that URL. You may use Powershell, Postman or any custom program to send messages to all users.

Please note - This will be bind to any specific Channel. You may create one Team which contain all the user whom you would like to serve the message.

Let me know if you would like to know practically how to achieve this.