Answer
Rather than trying to fit the full text into the Prompt label itself, use an event-driven stopping message to surface it: build a Custom Event that fires when the relevant condition is met (for example, a specific combination of other field values selected but the answer field still left empty), and have its action raise a blocking message using Error_SYS.Appl_General(...) containing the full long question text. This forces the complete question to be shown to the user as part of a validation message precisely when it's relevant (the answer field is mandatory but unanswered), working around the 30-character Prompt limit without needing a dedicated always-visible text display element.