This was removed for security reasons, but the closest alternative is to pass an expiring clone ticket as part of the URL, ex.
Replace "CLONE_TICKET" with the ticket from the MOB. Even easier, use PowerCLI 6.5 to generate a ticket and URL to log you in. See my post in the VMRC Shortcut thread for more information.
Edit: VMRC can also authenticate via WebSocket/MKS ticket. These strings can be retrieved via the acquireTicket API (vSphere 6.0 reference here).