JSFiddle - React, Tailwind, and code Playground

HTML

<select class="form-control" 
        data-bind="selectedOptions: SelectedCities,
                   options: $root.Cities, 
                   optionsText: 'CityName',
                   optionsValue: 'CityId',
                   valueUpdate: 'change'"
        multiple="true"></select>

<hr />

CSV: <p data-bind="text: SelectedCitiesCsv"></p>

JavaScript

var ViewModel = function() {
    var self = this;
    
    self.Cities = [
        {CityId: 1, CityName: "City 1"},
        {CityId: 2, CityName: "City 2"},
        {CityId: 3, CityName: "City 3"},
        {CityId: 4, CityName: "City 4"}
    ];
    
    self.SelectedCities = ko.observableArray([]);
    
    self.SelectedCitiesCsv = ko.computed(function(){
        return self.SelectedCities().join(",");
    });
};

ko.applyBindings(new ViewModel());