use the add to cart button to create the form elements, this will create the link.
You will need a column in the cart object for each posible sandwhich.
in the add to cart button, select each column, and set the Update able as Text Field option
after the form elements have been created in the addd to cart button form, edit the type attribute:
type="text"
to:
type="checkbox"
to convert them to check boxes.
make sure to give each checkbox a checked value.