After creating a Task Plan Item (e.g. Human Task Plan Item), open its specification.
By default “Synchronize with definition” is checked. Uncheck it.
The “Item control” field becomes enabled, select “Create Plan Item Control…”.
In the Plan Item Control Specification window that pops up, select “Manual Activation Rule” in the “Manual activation rule” field.
Click OK to all the pending dialog boxes.
You should be able to see the decorator.
Alternatively, since a plan item is associated with a definition, you can set the item control of a definition using above steps, then later created plan items can associate with this definition, without setting each plan item one by one:
Open Model Explorer by pressing Ctrl+Shift-O
The definition model element appears in orange, while the plan item appears in green.
Use similar steps as above, but this time update the “Default control” of definition not plan item.
When later created plan item points to this definition (need to enable by unchecking “Synchronize with definition” first otherwise you cannot set the definition of a plan item), it will inherits the item control properties as decorators.