When you use compact form fields, the field name or label will be displayed within the input box or text area. The text will disappear as soon as your form visitor starts writing in the field. This feature can be used for the following form fields: Short text, Long text, Number, Email, or Website.
After adding a field to your form, click on it in the main view to open the Edit Field panel. Under the label name, click on Visibility and select the inside the field option.