A custom field is going to show a value for each task. With a bit of fiddly work - you may be able to get it to work. For an example - set the Marked field for the "Go Live" date to "yes". Then in a date field:

IIf([Marked]=Yes,[Finish],ProjDateValue("NA"))

The formula should now show the date for the marked task.

Set the Calculation for group or summary rows to Maximum. Now that date appears at the Project summary task line.

Sorry - I can't suggest how to add the value to PWA. But yo should now have the DateX Project summary task value = Go live date.