If you click through from an email, the offer code is added for you.  If this is a percentage discount, all the products across the website are automatically discounted, so you will see the reduced price on the page and there is no need to add the code again. 

Please be aware that adding another offer code to the box will overwrite the email offer code, as you can only use one offer per order. Please also be aware that these offers are not available via cashback offer sites.  

If this does not fully answer your query and you are not able to find a relevant article, please raise a Support ticket using this link