Why is user information only saved when the form is resubmitted?

Question

PHP

Hello! There is a site on wordpress and a registration form in which (mandatory field) I want to receive the user's login by last name without displaying this field on the screen. To do this, in its value I put the value displayed in the last name field, but this option only works when you click the submit button again (for the first time, an error pops up that swears at an empty mandatory login field). How to fix this moment?

<form name="registerform" class="registerform" id="registerform<?php $template->the_instance(); ?>" action="<?php $template->the_action_url( 'register' ); ?>" method="post">
<p><label for="last_name<?php $template->the_instance(); ?>">Фамилия:<span class="necessarily">*</span></label><input type="text" name="last_name" id="last_name<?php $template->the_instance(); ?>" class="regular-text" value="<?php $template->the_posted_value( 'last_name' ); ?>"  /></p>
<p class="submit">
          <input type="hidden" name="user_login" id="user_login<?php $template->the_instance(); ?>" value="<?php $template->the_posted_value( 'last_name' ); ?>" class="regular-text" />
            <input type="submit" name="wp-submit" id="wp-submit<?php $template->the_instance(); ?>" value="<?php esc_attr_e( 'Register', 'theme-my-login' ); ?>" />
            <input type="hidden" name="redirect_to" value="<?php $template->the_redirect_url( 'register' ); ?>" />
            <input type="hidden" name="instance" value="<?php $template->the_instance(); ?>" />
            <input type="hidden" name="action" value="register" />
        </p>
</form>

Answer 1

Solution through scripts

<p><label for="IDLastName">Фамилия:</label>
   <input type="text" name="last_name" id="IDLastName" value=""  />
</p>
<input type="text" name="user_login" id="IDUserLogin" value="" />
<script>
jQuery(function($) {
    $('#IDLastName').keyup(function() {
        $('#IDUserLogin').val($('#IDLastName').val());
    })
});
</script>