How should a form properly and clearly handle and display a genuine server side validation error, such as discovering that a specific chosen username is already actually taken, that genuinely cannot actually be determined purely through simple client side validation logic alone?

Some specific validation checks, such as verifying that a given chosen username is genuinely still actually available, genuinely require making an actual real request to your backend server, and properly handling this kind of genuinely necessary server side validation typically involves submitting the form, awaiting the resulting server response, and if that server genuinely returns a specific validation error, properly mapping that particular returned error back to its own genuinely correct corresponding specific field and updating your form's own state to correctly display that returned error message, ideally reusing that exact same visual error display mechanism already used for your existing simpler client side validation errors.

final response = await submitForm(formData);
if (response.hasError) {
  setState(() => usernameError = response.errorMessage);
}