Using GDAP and PartnerCenter API in test Tenants

Question

I'm working on a school project where I have two tenants, main test tenant and a Contoso test tenant. I want to use the PartnerCenter to gain GDAP over the Contoso tenant. But it seems i need to be a Microsoft partner to be able to do so. I found a workaround by just adding an external user to the Contoso test tenant but than i'm not able to use the PartnerCenter SDK to approach the PartnerCenter API because I first need to register to an web App in the PartnerCenter.

I'm unauthorized (they don't show up) to use the sandbox and App Management in the partner portal