On the Editor toolbar, click the white box reading "write pop comment here", then type out your text. Hit enter for a new line (shift + return on some keyboards).
By default, the text is center aligned and the size of the box around it adjusts responsively as you type.
You can left or right align the text as well.
Copy the URL to this CSS file.
Click Configure in the project.
Paste the URL into the Custom CSS URL field.
This will set the default alignment to left. To set a specific annotation to right or center alignment, on the annotation, click Edit, More, Add Class. Then type