Spark - Calculate seats on signup page

Hello everyone,

I am currently working on a webapp that require me to calculate signup seats (to display pricing) before user signs up. The Spark documentation has this snippet:

Spark::chargePerSeat('Projects', function ($user) {
    return $user->projects()->count();
});

This works well when the user is logged in but I have a situation when the customer is already selecting a number of seats they want to order. How can I calculate pricing based on seats they are attempting to order, I tried:

Spark::chargePerSeat('Projects', function ($user) {
    return request('num_seats');
});

But the code never executes it seems as the user is not logged in.