@Supriya KM Please create a business rule as follows on sn_hr_core_case table.

Here I am assuming that location field on the user record has country field filled as Germany.

Here is the script

(function executeRule(current, previous /*null when async*/ ) {

    // Add your code here
    var watchlist = current.watch_list = '';
    if (watchlist) {
        current.watch_list = watchlist + ',' + current.opened_for;
    } else {
        current.watch_list = current.opened_for + '';
    }

})(current, previous);

Please mark the response helpful and correct if it manages to address your issue.