You can create a Read Extent Action through an Activity Diagram or a Modeler pane.
To create a read extent action through an activity diagram:
1. On the Activity Diagram toolbar, click the arrow in the split button that is to the right of the Accept Event Action button, click the Read Extent Action button, and then on the Activity Diagram, click in free space or within an Activity Partition, Interruptible Activity Region or Structured Activity Node.
2. From the Select Object dialog, select the Class, Data Type, Interface (for Read Extent Action only) or Signal that is to be the Linked Item, and then click OK.
3. Type the name of the Read Extent Action, and then press the Enter key.
Modeler creates the Read Extent Action and on it creates the Result Pin output pin.
To create a read extent action through a Modeler pane:
1. In a Modeler pane, right-click the Activity or Structure Activity Node that is to own the Read Extent Action, point to New, point to Action Node, and then click Read Extent Action.
2. From the Select Object dialog, select the Class, Data Type, Interface (for Read Extent Action only) or Signal that is to be the Linked Item, and then click OK.
3. Type the name of the Read Extent Action, and then press the Enter key.
Modeler creates the Read Extent Action and on it creates the Result Pin output pin.
Accept Event Action button, click the 
Read Extent Action button, and then on the Activity Diagram, click in free space or within an Activity Partition, Interruptible Activity Region or Structured Activity Node.