To pass an event parameter, you have to map it to an event handler parameter. This is done using IWDParameterMapping. You can map existing parameters like the index of the DropDownByIndex that contains the index of the selected item. You can also add a parameter to an event and transfer a constant value to the corresponding event handler. This case is described here.
You have created an action with a parameter.
1. To create a constant value for the event parameter, select the UI element in the Outline view, open the context menu and select Parameter Mapping.
2. Create a constant value and confirm with OK.
3. You can now see the constant value of the new event parameter. It is mapped to the event handler parameter, as shown in the screenshot below:
If no arrow appears, select the parameter on the right hand side and map the event handler parameter to the constant value using drag and drop.