Agentry - storing a value passed into an action

This should be easy to do although I can't seem to find the right means to do it. I want to take the value passed into an action and store it in a property of MainObject. I should be able to do this in an action step that uses a transaction, however in the property of the transaction (which targets the MainObject property in which I want to store this value) I can't find a way to get the value passed into the parent action which is calling the transaction.

Thanks

Tim