How to: Add GUI to menu in IntelliJ?

First of all: Good morning / Good day / Good evening
Pick whatever fits yourself best.

I'm currently building a GUI in wich I want my plugin-functionalities to be configurable.
Now I want to embed my GUI into the menu on the bottom wich also holds "TODO", "Version Control", "Terminal and so on.

I would think it works a bit differently from just adding a menu with some actions like you do in the main menu.