Hi Deepu Kumar,
The easiest why to create custom monitor, is to actually browse an application and inspect the traffic (requset/response) via the developer tools which has any modern browser, or you can do the same via CLI using curl utility.
For example in Firefox (which I generally use), you have to open Web Developer Tool (Ctrl+Shift+I), then you have to switch to Console tab, Filter the Requests and analyze requests/responses, but in raw format, to see how actually browser sends request and what it actually gets from web-server before interpreting it to you visually. It gives you more visibility and clue how to write your own custom monitor.
I can just hint you, in your case, that you can create https monitor, where in send string, you can put something like this:
GET /abc/authenticated.aspx HTTP/1.1\r\nHost: yourserver.example.com\r\nConnection: close\r\n
In the Receive String, you can write exact response string, or just regular expression which the system will try to search in response body.
That's it, you don't have to configure invalid response, as if the response will not match valid response, it'll automatically marked like invalid and that pool member will be set as unhealthy.
If you have 4 different web applications, then you have to create 4 different monitors for each.
Hope that helps.
// Giorgi