how can i enter my licence key to activate on new laptop
It depends which version of CCleaner you have installed.
For CCleaner v6 it’s Options>License key.
For CCleaner v7 it’s Settings>My Subscription.
Or do you mean that you want to transfer your existing licence from one laptop to a new one?
To do that simply install CCleaner Free, v6.39 or v7 is your choice, and go to the relevant page as above to enter your key and activate it as Pro.
You can find installers for both CCleaner7 and the ‘Sunset’ v6.39 here:
Once you have activated the CCleaner on the new machine as Pro, then unless you are covered by a multi-device licence, you should uninstall if from the old machine.