How to add a computed observable via prototype

HTML

<script src="http://knockoutjs.com/js/knockout-2.0.0.js"></script>
<div data-bind="text: fullDomainName">test</div>

JavaScript

function SiteModel(rootUrl, data) {
    var self = this;
    self.rootUrl = rootUrl;
    self.DomainName = ko.observable(data.DomainName);
    self.IsSubDomain = ko.observable(data.IsSubDomain);
    self.fullDomainName = ko.computed(self.fullDomainName, self);
}

SiteModel.prototype.fullDomainName = function () {
    if (this.IsSubDomain() && this.DomainName()) { // bombs out here with "self.IsSubDomain is not a function"
        return this.DomainName() + ".myCompanyWebsite.com";
    }
    else {
        return this.DomainName();
   }
}; 
 
var temp = new SiteModel("someurl", { DomainName: "extraCool" });

ko.applyBindings(temp);