On your wedding website, info cards, etc, but I wouldn't place it on the invite.
Also, I suggest reading your crowd to see whether this is a good idea or not in general. Some crowds do not like things like honeymoon funds and are downright mean about it; others are fine with them. And others yet think this is a great idea!
If you're not sure, or expect a negative reaction, another option is to just have a very small registry (or none at all). People usually just give cash instead. In this option, you'll get a lot more questions directed to the family, wedding party, etc. about what you want. Just make sure they know. Our wedding party told guests, "cash so they can save for a house and buy a fancy gaming table." That's exactly what we got.
We didn’t register and so far out of over 70 people invited, only one has asked FH if we are registered anywhere. We just know our crowd really isn’t into that. What’s more, it makes me feel kinda gift grabby. I don’t know how much anyone can afford on a gift and I don’t want them to not come to the wedding because they can’t afford something on the registry. My cousin had a small wedding a couple of years ago and our family members showed up with cards and money.
I kinda figured that would happen at our wedding as well. FH’s side also seems to do money and not gifts. I will be thankful and appreciative of anything given to us. If we are given money, it will go toward a fund to save for buying a house.
I told my fiancé I want to put a honeymoon fund box next to our guest book but he said that it’s embarrassing so I guess that’s a no. We’re planning to buy a house so he wants to do a registry instead but I think receiving a monetary gift is better especially with our expenses for the wedding adding to the list. We have this money dance that we do every wedding in our culture (Filipino). I’m thinking of adding that to the program if he doesn’t want to put a box.
It's cute but I still don't know that I would add that to the invitation anywhere, even in an insert. It could go on your website, but I would advise against putting it in the invite. Then again, it's a know your crowd thing.