There are many questions and doubts that you ask us about obtaining this credential. The staff of the Office of the Pilgrim, always so diligent, makes it clear that they have no such credentials as the Camino de Santiago in Santiago just exactly, but we have two options if you want to continue to Finisterre or Muxia.
The first is that if we still have space sealing the credential that bring the French Way, Portuguese etc.; and the second is that this credential can ask the Tourist Office in Santiago, where if available, or in one of the hostels to reach Negreira.
So you can get on top of La Compostela, La Fisterrana o muxiana, which demonstrate that you have reached the end of the world.