The main input box is used for adding or editing the action code. Pressing the Return key commits any changes made and runs (or updates) the query. The code box has auto-completion for action code and attribute names (based on using a $-prefix).
Note that an agent's query, action and priority can also be set from the agent tab of the Get Info pop-over.
From v7.2.0 OnAdd also affects adornments created or pasted into containers.