This particular nugget is missing from both the LW and the Parcelforce documentation so figured I'd fill the hole for the next person desperate for the information.
Credit: Phil Davies for the assist
Accurate as of: 6th August 2018
Once it's all up and working you then print your PF labels as normal via LW
Booking the Collection
Now, assuming you know you are going to need an adhoc collection for that day it's best to get this in nice and early so you're sure the driver knows to come (our driver said book before 1pm - yours may be different)