Coupons can now be set to be automatically applied on the checkout page which saves the user from having to enter the code into the coupon code field.
To enable this, go to Site Settings > Prices & VAT > tick Enable support for auto applying coupons at checkout and then Update Settings.
To use this, add the coupon as normal and then in Assign Coupons tick the new column called Auto Apply.
The coupon will then be automatically applied on the checkout page, using the normal coupon settings you have used.
Remember to put a test order through the checkout to make sure everything works fine.