Hello @aydoğan ilhan ,
You couldn't send the notifications to other applications. You can only send the notification to yours.
When a user presses the button in your APP, you can send messages with your app server, then send messages to the other application via server. The premise is that the two servers can communicate with each other, and you have configured the push notification/Remote notification in the other application (both server side and client side).
For Xamarin (client side), you could see - Push Notifications in iOS - Xamarin | Microsoft Learn and Firebase Cloud Messaging in Android - Xamarin | Microsoft Learn
If the answer is the right solution, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.