In an ArcGIS Online web map, if a valid URL exists in an attribute field value, the user sees a live hyperklink labled "More Info" from which they can link to an external web page. It would be very handy if this same thing could happen based on a calculated value in a popup when using the custom attribute display option. The author would likely need an option to insert a string into the custom attribute display configuration panel to explicitly declare insertion of an item that is calculated and that the calculation result is intended to be a live hyperlink. This sort of capability would also be handy in the regular attribute list configuration. In that case, you might consider just having a way to add a calculated value to the list of attributes to be displayed in the popup.