Answer
This can't be changed through configuration — it requires code. The message-box call takes a flags parameter that includes which button is the default (Sys.MB_DefButton1, 2, or 3, alongside the icon/button-set flags like Sys.MB_IconInformation | Sys.MB_Ok); passing the appropriate default-button flag when constructing the message box sets which option fires on Enter. There's no end-user-configurable setting for this — it has to be built into the message box call itself.