Have you started the app and opened the page? What you are seeing in the modeler is the Attribute name and the label. The Attribute will be resolved to a value when you run and view the app.
What you want is not a “default text” but a "Placeholder text”. A "default text” is the attribute's value that it is given directly upon creation of the object it belongs to. And if it does not get changed, it will be stored in the database.
A “placeholder text” is what you describe, the text shown in the attribute's box, which disappears when the user types anything in the box. You define that on the page at the attribute's property:
And it looks like this: