A 'Message' construct can be added to a 'Sequence Diagram' via drag-and-drop.
Within a 'Sequence Diagram', select the source 'Lifeline' where you would like the 'Message' to originate from.
Click the green circle and continue to hold down the left mouse button.
Drag the green circle to the dashed line below the destination 'Lifeline' of your choice.
When the dashed line below the destination 'Lifeline' highlights green, release the left mouse button to drop the new 'Message' and add it to the diagram.
Notice the 'Message' stays selected once it has been dropped. Since it is selected, the toolbar changes to include buttons for functions which can be used on the construct. The sidebar also changes to include additional 'Metadata,' 'Attributes,' and 'Relationships' tabs.
Once added to the diagram, select your new 'Message' and enter a meaningful 'Name' for your 'Message' via the 'Attributes' tab of the left sidebar.
Click the 'Save' button located on the toolbar to persist your changes to your project's database.
Please Note: The above process describes using the 'New' tab of the left sidebar, which automatically generates a new entity to represent each new diagram construct. If you would like to reuse existing entities from your database to represent a new construct, use the 'Existing' tab instead.
For more information on this construct and how it is represented in the database model, see Sequence Diagram Constructs.