Need to capture Logged in User details in Site

Hi Everyone, I have a requirement where I need to push Logged in User details immediately when user accessed a Site and when no other action (like clicking any tabs) is done by User.

We have a custom Smart Service built for pushing the required data to third party system and is being executed in a Process Model. We are able to push the other required data either through Interface (using function a!startprocess()) or process model(using Sub-process call). But we are not able to capture the Logged in user details as there is no way to call the Smart Service or process model at SIte when user first accessed it.

Could you please provide your suggestions on possible ways to achieve the requirement. Also please suggest if any alternate approach is there. Thanks in advance.