gistfile1.txt

add_action( 'register_form', 'add_my_field' );
    function add_my_field(){ 

     $custom_field = ( ! empty( $_POST['custom_field'] ) ) ? trim( $_POST['custom_field'] ) : '';

       echo '<label for="custom_field"'.  _e( 'Custom Field Label =', 'textdomain' ) .'</label><br />
                <input type="text" name="custom_field" id="custom_field" class="input" value="'. echo esc_attr( wp_unslash( $custom_field ) ) .'" size="25" />
              ';
} 

add_filter( 'registration_errors', 'custom_field_sanitize', 10, 3 );
    function custom_field_sanitize( $errors, $sanitized_user_login, $user_email ) {
        
        if ( empty( $_POST['custom_field'] ) || ! empty( $_POST['custom_field'] ) && trim( $_POST['custom_field'] ) == '' ) {
            $errors->add( 'custom_field_error', __( '<strong>ERROR</strong>: You must include a value.', 'textdomain' ) );
        }
        return $errors;
    }

add_action( 'user_register', 'custom_field_save' );
    function custom_field_save( $user_id ) {
        if ( ! empty( $_POST['custom_field'] ) ) {
            update_user_meta( $user_id, 'custom_field', trim( $_POST['custom_field'] ) );
        }
    }