Trendline alerts differ from the other alerts in that they are not entered or edited in the Alert Editor. In order to create a Trendline Alert, you have to draw a trendline annotation on a chart. Moving the trendline on the chart will change the alert.
To distinguish alert trendlines visually from regular line annotations on the chart, they will always be dashed lines in two colors, main and interleave, unlike all other annotations.
Once you draw the trendline alert, you can edit its alert parameters - right click on the trendline to bring up its parameters editor, and click on the EDIT Alert button in the editor. Doing that will bring up the alert editor where you can change the alert parameters for this particular trendline.
In the alert editor, you can change whether the trendline alert will get triggered on the crossing up or down (or both) and which price parameter triggers it. You can also, as for any other alert, change its notifications and note.
Deleting the alert trendline will delete the corresponding alert and vice versa - deleting the alert in the alert editor will delete its trendline.
NOTE: The chart does not need to be opened in order to monitor and trigger the alert. But the quotes for the symbol have to be coming in - either because a portfolio that contains this symbol is open and active or because another chart is opened and is pulling in quotes.