通过原型向构造函数添加可观测的计算值

I'm using Knockout.js 2.0 and I'm trying to extend the prototype of the constructor function I've created by adding a computed observable but its throwing up "self.IsSubDomain is not a function". How do I solve this error? Is there another way to extend a constructor function to solve this?I'm using Knockout.js 2.0 and I'm trying to ext