OAuth without login

Hy everyone,

I'm making an API for Exact Online for a website with a form. The visitor will fill in his information and after that the visitor sends it, it need to be send to the Exact online account from my client. But before that I need a accesstoken. The problem is that I don't want to give the user the login page that Exact will send me. I'm searching already days for a way to skip that login or to enter the login information by backend (there is always 1 login, and that is the login from my client).

Now this authorization thing is something new for me. So far I know I can call my authorization settings from the startup with this:

HttpContext.Authentication.GetAuthenticateInfoAsync("ExactOnline");

But then I get that loginscreen that I don't want. The only thing that Exact is telling me to do:

Create an app registration that supports an automated connection wizard (your provisioning process).

Is there a way to send them the information and the visitor doesn't see a loginpage.

In my Startup.cs

var s = new OAuthOptions
{
    AuthenticationScheme = "ExactOnline",
    ClientId = "CLIENTID",
    ClientSecret = "CLIENTSECRET",
    CallbackPath = new PathString("/callback"),
    AutomaticAuthenticate = true,
    AutomaticChallenge = true,
    AuthorizationEndpoint = new Uri(string.Format("{0}/api/oauth2/auth", "https://start.exactonline.nl")).ToString(),
    TokenEndpoint = new Uri(string.Format("{0}/api/oauth2/token", "https://start.exactonline.nl")).ToString(),
    //Scope = { "identity", "roles" },
    Events = new OAuthEvents
    {
        OnCreatingTicket = context =>
        {
            context.Identity.AddClaim(new Claim("urn:token:exactonline", context.AccessToken));
            return Task.FromResult(true);
        }
    }
};
app.UseOAuthAuthentication(s);

Also another question: What is the best place to store the accesstoken and refreshtoken.