Add Field:

Opens a window in which you can select a database field to add to the form or report.

The field selection window lists all database fields of the table or query that was specified as the data source in the Form Properties.

You can incorporate a field into the current document by dragging-and-dropping it into the document while holding down the mouse button. A corresponding field is then set up in the document with a link to the database.

If you add fields to a form and you switch off the Design Mode, you can see that LibreOffice sets up a labelled input field for every inserted database field where the user can enter data.