Add or modify a trigger
When you add an action to the artboard in Microsoft Expression Blend, an EventTrigger is created by default. You can use any of the additional built-in triggers in Expression Blend by changing the trigger type.
To change a trigger type
In the Trigger category, next to TriggerType (EventTrigger), click New. In the Select Object dialog box, click the trigger that you want, and then click OK.
You can also double-click the trigger that you want in the Select Object dialog box to select it and then close the dialog box.
You can modify the trigger properties in the Trigger category of the Properties panel according to the trigger type that you have selected.