Showing A Flash Message On Admin Entries Listing (Craft 3)
To show a message 

Craft::$app-&gt;getSession()-&gt;setNotice($message);


To show an error

Craft::$app-&gt;session-&gt;setError($message);


There is no "default" key that is always shown or something like that. You just use the function and set a message -> the next time the user visits any page the message is shown.