In this code example, the custom setting will be displayed below the default value option when a star field type is selected.

add_action( 'frm_default_value_setting', 'after_default_value_setting' );
function after_default_value_setting( $args ) {
	$field = $args['field'];
	$value = isset( $field['default_value'] ) ? $field['default_value'] : '';
	if ( 'star' === $field['type'] ) {
		?>
		<label>
			<input type="checkbox" value="1" name="field_options[custom_setting_<?php checked( $value, 1 ); ?>]" />
			<?php esc_html_e( 'Custom setting', 'text-domain' ); ?>
		</label>
		<?php
	}
}