Process Services Enterprise API 2.0.0 - Authentication

[mhassancs77]

i want to authenticate  obtaining the token  using the API 
i am tring to call this api : http://localhost:9999/activiti-app/app/authentication
with the following request body

"j_username": "admin@app.activiti.com",

"j_password": "admin",

"_spring_security_remember_me": true,

"submit": "Login"

but it gives me 401 not authorized

although the username and password are correct
i used it to login on the website on the login form