I'm new to developing & I'm working on a checkout page and need to be able to select a saved address from a combo-box and then once selected, it needs to populate hidden fields. I've tried a few things but can't find one to suit what I want to achieve.

Checkout Page - https://i.stack.imgur.com/Cks54.png Hidden fields it needs to populate - https://i.stack.imgur.com/UggTd.png

The hidden fields are stored in a layout group and are unhidden using the following function;

        function ShowBillingFields() {
        var BillingDetails = flBillingDetails.GetItemByName('lgBillingDetails');
        BillingDetails.SetVisible(true);
    }