Pre-filling is set on the form type: Settings → Observations / Progress / Forms → Form types → pencil icon of the type.

Fields of a form type with their default values and the dynamic prefill menu
Default values and dynamic tokens of a type.

For each field of the PDF, enter a default value. The “Dynamic prefill” menu inserts tokens that are resolved when each form is created:

The values are set when the form is created and can be changed afterwards. A form that already exists is not filled in again.