Use this code example to display the input without saving the setting.

add_action( 'frm_misc_settings_form', 'custom_misc_settings' );
function custom_misc_settings( $settings ) {
	$value = isset( $settings->custom_setting ) ? $settings->custom_setting : '';
	?>
	<p>
		<label for="your-custom-setting">
			<?php esc_html_e( 'Your custom setting', 'text-domain' ); ?>
		</label>
		<input type="text" id="your-custom-setting" name="custom_setting" value="<?php echo esc_attr( $value ); ?>" />
	</p>
<?php
}