Enter Annotation Mode after that make a rectangular shape on the signal area where you want to add an annotation.
Then select an event concept from the drop down list or from Favorites (which is below the list)
If user has any notes or remarks he can type them in additional notes field.
Last step is to press Save button.
You can exit annotation mode by hitting ESC key on your keyboard or clicking x button on Annotation menu or by clicking with your mouse on other signals besides annotated one.