You can link them through MDE up to 30 days in advance of your visit, at least tht is my understanding. SOG is supposed to be sending my ticket reservation request for this week. Once I pay for my tickets, SOG sends me the ticket numbers and I can then make FP+ reservations 30 days out.
Right, I used the voucher numbers and got 60 days out because we were staying onsite. If you're staying at SOG or offsite, it's 30 days. Easy, like said above. What you can't go, is enter a park until you activate the voucher at guest services outside the park or in DTD.