Your mission is to login as Admin.
The application is vulnerable to sql injection, but the signup process seems a bit "weird".
I am sure you can login as Admin somehow.
It is also possible to register via the login form. Simply enter a new username and your desired password.

Good luck!

Hint: You know the table is called "users" and which holds the columns "username" and "password".

Thanks go out to Jinx for motivation and idea.