Automatic registration when ordering. How to add a birthday date to a user profile?

Question

1C-Bitrix

Good afternoon!
Added string property Date to order properties. The mnemonic code is BIRTHDAY.
How to add this field to the user account during automatic registration?
I use the event handler OnAfterUserAdd

AddEventHandler(
      "main",
      "OnAfterUserAdd",
      "OnAfterUserAddHandler"
    );

    function OnAfterUserAddHandler(&$arFields) {

      if (  !empty( $arFields["BIRTHDAY"] )  ) {

        $user = new CUser;
        $fields = Array( "PERSONAL_BIRTHDAY" => $arFields["BIRTHDAY"], );
        $user->Update( $arFields["ID"] , $fields );
      }
    }

Answer 1

There is an elegant crutch. On the order creation event, write the date to the session, for example, in the field REGISTER_AND_LOGIN_PERSONAL_BIRTHDAY, on the prologue event, check for non-emptiness of this field in the session, if it exists, then write it to the user account and delete this field from the session.