Create a new Calculated field that will evaluate if field is empty. You need to disable that beneath the calculation window.
IsEmpty( date::closed ) ; "Ongoing";
You can't add text to a date field, but you can get what you want.
In FileMaker 15, Enter "Ongoing" into the Placeholder box in the Inspector
In older versions add "ongoing" to the text and use Hide Object When with: Not Isempty (yourfieldhere) to keep the text invisible if there is a date in the field.
In even older versions, you can use conditional formatting to format to a very large text size (300+) to make text disappear.
HAHA, Thanks Phil. I recently purchased FMA15 and didn't even know that was there.