Display tex in numeric field

Hi, I'm totally new to Adobe Designer!

I want to create a form, where the user are required (value type: "User Entered - Required") to enter monitoring data in numeric fields. However, it is possible that there is no data available for entering so the value is different from "0".

For this I want that the field displays "missing data". I don't know how to do this. The field will alway only display "0" when entering string (e.g. "missing data") or entering nothing.

Somebody help me please / thanks a lot. Wilko