The way MS give the unique app id is quite messy.
Actually MS release the guid only during the first app binary submission, not before when you was creating the MS Store page.
So, you should publish a dummy application, hide it and make it not showed to the users.
After that MS will provide you a guid you could enter during activation key generation.
NOTE: we all know that the MS process is crazy and if you will need to regenerate your key due to mistakes or misunderstandings you could create a ticket here
addressing to licensing.
Please, remember to enter your order reference number while creating the ticket.