add_action( 'frm_after_field_options', 'add_custom_field_options_section' );
function add_custom_field_options_section( $args ) {
	if ( 'text' !== $args['field']['type'] ) {
		return;
	}
	$value = isset( $args['field']['custom_setting'] ) ? $args['field']['custom_setting'] : '';
	?>
	<div class="custom_field_settings">
		<h3 class="frm-collapsed" aria-expanded="false" tabindex="0" role="button" aria-label="Custom settings" aria-controls="collapsible-section">
			<?php
			esc_html_e( 'Custom settings', 'text-domain' );
			FrmAppHelper::icon_by_class( 'frmfont frm_arrowdown6_icon', array( 'aria-hidden' => 'true' ) );
			?>
		</h3>

		<div class="frm_validation_box frm-collapse-me" role="group">
			<p>
				<label>
					Custom setting
					<input type="text" name="field_options[custom_setting_<?php echo intval( $args['field']['id'] ); ?>]" value="<?php echo esc_attr( $value ); ?>" />
				</label>
			</p>
		</div>
	</div>
	<?php
}

add_filter( 'frm_default_field_options', 'add_custom_field_option', 10, 2 );
function add_custom_field_option( $options, $args ) {
	if ( 'text' === $args['type'] ) {
		$options['custom_setting'] = '';
	}
	return $options;
}