Add Order Discount Manually

Anyone know where I would start to manually add discount to orders. So I add the order from the backend manually I can add the billing add the items calculate the totals and edit the Order Total with the pen but I cannot edit the discount? Anyone done this I cannot get a hook that is anywhere near this I am aware I can add a minus fee but ideally I want to apply this to the Discount amount so I can use the meta later. A pen next to discount to load a field the same as Order Total is ideal but appreciate this is probably not achievable. I was thinking maybe another button in the Add Item buttons but I cannot see a hook for this?

Thanks in advance
J