I don't have a premium account to test, could you PM me the login details to test.
Edit:
In their "api/upload/server" response they don't return any utype (that's why it's not added). I can't just guess if it's prem or normal user.
They need to return utype with "api/upload/server" response (that's how all other file hosts do and that's already supported).
I also even tried to manually add "utype=prem" to request, but then they just show:
Code:
500 Internal Server Error
nginx/1.10.3
They need to solve these issues at their end.