A 'Lifeline' construct can be added to a 'Sequence Diagram' via drag-and-drop.
Within a 'Sequence Diagram', click the 'Lifeline (Asset)' icon in the 'New' tab of the left sidebar and continue to hold down the left mouse button.
Drag the 'Lifeline (Asset)' icon over to the adjacent diagram canvas.
Release the left mouse button while over the diagram canvas to drop the new 'Lifeline' and add it to the diagram. The 'Lifeline' will automatically snap to position at the top of the diagram, ordered by number.
Notice the 'Lifeline' 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, enter a meaningful 'Name' for your new 'Lifeline' via the 'Attributes' tab of the left sidebar (focused automatically for convenience).
Click the 'Save' button located on the toolbar to persist your changes to your project's database.
* 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.