Hello,
As of R2022b, you can add plain-text annotations to a sequence diagram to describe elements, such as lifelines, messages, and fragments. To create an annotation, double-click the canvas at the desired location. Then, enter the annotation text in the text box that appears on the canvas. Press Esc or click anywhere outside the text box to apply the changes.
For more information, refer to the documentation link given below:
Please note that this functionality is introduced in R2022b and is not available on R2022a and earlier versions.
