Gets or sets the fully qualified name of the signature field to sign. An unsigned signature field of that name is signed; when
the form has no field of that name, a new invisible signature field is created under it. null (the default)
creates a new field named SignatureN, N the lowest number from 1 the form does not use.
public string? FieldName { get; set; }
ISO 32000-2 §12.7.4.2 (field names) and §12.7.5.5 (signature fields; invisible when the widget's rectangle is empty).