Dynamic content is a reference to a value produced by the trigger or an earlier action. In an email-arrival flow, the sender, subject and body belong to the current event. Select the appropriate token in the field that needs it. The designer may show a friendly token, while its expression refers to a particular property.
This simulator uses expressions such as @triggerBody()?['subject'] to keep those references visible. The question mark allows a missing property to produce a missing value rather than failing the property lookup. It doesn't invent a replacement address. Decide explicitly whether a blank is acceptable. The hidden checks change names and message text, so typing the visible sample value will fail. A flow should produce the correct effect for the next event as well as the first one.