Looking at the source for Console Proxy I solved this problem per below :
Use the same authentication procedure the normal cloudstack api use defined her - http://docs.cloudstack.apache.org/en/latest/developersguide/dev.html
Use the same API Key and sign the request with API Secret provided from Cloudstack:
The Console proxy endpoint are as follow - /console?cmd=access&vm=xxx, on success a Url will be returned with a token to access the virtual machine.
Test Results -
Request:
CloudstackRequestClientFilter -> cmd=access&vm=29603248-6d8a-4582-aa9a-4d1bfb4d7714&apikey=3NRrdrhDTwggQ_oQny11dD39-XRWJxCd0dh2xqtMNShrz_jb4ZdhHtmRh7NYiOfRzLNwPcBVAfT9FHh9v96vzg&response=json&signature=u4c7QZNQNcN+2s3fhRNSHTyl7+Q=
Response :
frame src="http://172.16.90.99/ajax?token=TCbfnguNvsHEkga3jPJEfZctqiPHTEynM6sAG2K8iIuioKHU8UU1QAWuQLHATd0dznP9vXPggHJp9km_1bmmStiD1PPKr6nZeid0NVI7kUt8_vOGkOK4vdM2d388KFj8oA280mQ-ZjPHWPgU4gCn47nLVb-2cVxNgOijOjdgDEDj5vlqFkzz2YhcqkLt6CIVdFcAJ1g1gqvhrO530ubjLZsiQvxs_kn9X8eXMafhRm_qugu3k6lLuG38zXsK9jKNWkmqoAV2EBBZh-r6agm4dQ">
Hope this helps, took met sometime to figure this out, source for the console proxy can be found here :
https://github.com/apache/cloudstack/blob/master/server/src/main/java/com/cloud/servlet/ConsoleProxyServlet.java