how to add a custom field such as TAX number on the checkout page? I can’t seem to figure it out as I need this as an optional on the checkout page.
Hi!
The only ways to customise the checkout page is by going to Admin Panel → Settings → Checkout.
Those are the only options you have to customise that page, i don’t believe you can add custom fields unfortunately 
Hi, I came across these detail steps to add custom attributes on checkout page. Hope this helps to solve the problem.
That is to add custom fields to your Cart page not your Checkout page unfortunately. Editing of the checkout page isn’t possible to the extent of adding new custom fields