How to implement a flag popup with java method condition

Actually I have a IssueEventListener implemented in JAVA. Now, based on the condition I am checking within the EventListener, I want to display a flag (AUI) to the user.

Simply, this is an OutOfOffice plugin in Jira which checks, if a user is in vacation and if so display a flag to the user, that he should be aware, that the user is not in house. This check is done with reading a txt file with the OOF information.

How can I create a AUI flag UI and display it based on the condition checked with the IssueEventListener?

Thank you in advance.