Here are all the codes for that model. Send the document to Logitech support noting the commands you want added. They will convert to hex, so you don't have to worry about that. I'm pretty sure your 890 is well out of support, so you'll probably have to pay. A free solution would be to use a cheap/free JP1 remote to teach your Harmony. By far the simplest method would be to simply load similar Denon models until you found one with the commands you wanted. Most Denons of that vintage use the same codes.

[Link: awe-europe.com]

If you want to convert to hex yourself, download and install IR scrutinizer and I'll advise you.