Solved
Adobe Campaign Standard: How can we set attribute value to submission datetime on submit of button on LP in ACS?
We tried setting using Additional data but looks like ACS treats values in Addtional data field as text, so can't evaluate GetDate() function.
Additionally, tried luck by setting the value through document controller (the way it can be done in AC classic), but even that is not working.
function setSubmissionDate()
{
var date = new Date();
console.log(date);
document.controller.setValue('/context/profile/OptoutDate', date);
}