return error from form

Hi,

I'm trying to use Language files in CI4 for (amongst other things) customs errors. However, I'm not sure how to construct a response to a form submission with a customer error?

PHP Code:

if(empty($a) && !empty($b)) {
return response()->with('error', lang('myfile.AisRequiredWithB'));
} 


I'm sure there is a simple process to achieve this?

Any assistance is appreciated