You can create a new file with the extension .ros2 in the rosnodes folder from the previously created project by right-clicking in the folder and selecting New -> File.
This will create an empty file where you can create your model. As a helper, you can use the reference documentation and press Ctrl + Space bar to use the auto-complete functionality.
If auto-complete or the grammar highlighting is not working, check that the file is open with the "Ros 2 Editor" with a right-click on the file and under "Open with..".