Does anyone know how to do this please? I can see how to add litters, which I don't obviously need to do right now. I can't see how to add planned litters? Thank you! :)

You can't do this. You can only add a litter after the breeding when you have a due date.

You can make a statement on your CD page that you are planning a litter, but you can't create a litter entry.

Taken from the terms and conditions https://www.champdogs.co.uk/info/conditions 
•You may refer to previous and forthcoming litters on your main page, but the details of all litters must be entered on the Litter Register.

Thank you, I have actually mated my bitch, a few weeks ago. I just thought you could add "planned" litters.

@Lexy, thank you! That is what I thought...

Put in the due  and it should be ok