Is it possible to supply a default text value for the "text" daypilot modal form

Yes, the form will load the initial values from the source data object using the property mappings (id property of the form items).

The source data object (data in the example below) is optional. When editing an existing item, you will want to use the existing data. When creating a new item, you can use it to pre-fill the default values.

const form = [
  { name: "Text", id: "text" }
];

const data = {
  text: "Default text value"
};

const modal = await DayPilot.Modal.form(form, data);

// ...