Add new user : make the fields First Name and Last name required

I’m searching how to make First Name and Last name fields required when we add a new user. Right now only username and Email fields are required.

I found a way by adding class="form-required" for the first and last name fields on the file user-new.php.

But I’m looking for a method with adding code on function.php and not touch to the WordPress Core.

Thanks.