I usually note it in the code box something to the effect of "No code provided my mturk user id is xxxxxxxx" Like TSolo315 said, you can e-mail the requester if you want. I have never had any problems with just putting my MTurk Id in the box when there is no code.
I wouldn't mention it in the code box, personally. I't submit with my AMT ID in the code box. If there's a comment spot, I'd put that comment there. If there isn't, I'd use 'Contact requester' with the message, "Just completed your study. No code given at the end so, submitted with AMT Worker ID. I have a screenshot of the last page of the HIT, without the code, if that helps. Thanks in advance for preventing this from becoming a rejection. Thanks for being a requester on MTurk. Have a good day!" The reason I don't suggest putting it in the code box is because, if they try to solve their problem by automating the entries in the code box as a list of ID's to approve, they'll have to manually remove the additional stuff in the code box on yours, which could slow or foul the process.
But, always submit. I'd rather have the opportunity to get paid for my time, with the super small risk of a reversible rejection, over making the decision to be guaranteed not to get paid by returning it.