residence permit no. Youth mobility visa ?

Help! I've started the process of applying for the Youth Mobility Visa for Germany. I am filling in the Application form (Antrag auf Erteilung eines nationalen Visums) but I am puzzled by one thing. They ask for a Residence permit No and I am not sure what this is referring to? I am applying from my country of residence (Canada) Could someone guide me: Do I possess this no ? And if so, where can I find it?