Add meta to cart

I have added a deposit field to the booking form.
– Added it to class-travel-tab-tour.php and have it saving to the db in admin
– have it showing on the booking form (booking.php) and it is being recalculated with qty change
– I added a hidden field with the variable to the form

I just can’t get it into the cart data – it is not in the meta
Do you know how I can add it to the session?

Thanks