Android 5.5.1 How to display a message after the system starts without writing an application?

Question

Android

Actually, the problem is in the title.

The simplest way, in my opinion, is an application that, when launched, displays a message specified in its settings. Such an application itself would be launched by an autorun application. I found an application for autorun on the play store, but an application that displays a message when it starts is not.

I will be glad if you tell me such an application, or another simple way to solve the problem. Thanks in advance.

PS By message I mean a dialog box with an "OK" button.