My app has background service which keep on running even if app is killed from the task manager but when I have tested my app on MI(xiaomi) phone it kills my app service whe
you are not able to do that from code until and unless there is some api from xiaomi that gives you access to that functionality. I am guessing auto start manager is an app( with privileges of that of a system app, since part of xiaomi modified os) , hence not possible.
But if the device is rooted you can actually disable autostart manager.
Still whatsapp , facebook and many more apps doing this may be because of they have tie ups with Xiaomi to be in the whitelist. But its just a guess.
You should select release build variant from android studio because MI has disabled debugging mode.
MIUI has a built-in Security app. In the Security app there is a 'Autostart' section, where the user can configure which app can and which app can't run on startup. Your application is disabled default. You need to enable your app and also enable permission manager from permission section.