How do you consume extra parameters in OAuth2 Token request within .net WebApi2 application

As it often is the case, I found the answer immediately after submitting the question…

ApplicationOAuthProvider.cs contains the following code out-of-the-box

public override async Task GrantResourceOwnerCredentials(OAuthGrantResourceOwnerCredentialsContext context)
{
    using (UserManager<IdentityUser> userManager = _userManagerFactory())
    {
        IdentityUser user = await userManager.FindAsync(context.UserName, context.Password);

        if (user == null)
        {
            context.SetError("invalid_grant", "The user name or password is incorrect.");
            return;
        }

        ClaimsIdentity oAuthIdentity = await userManager.CreateIdentityAsync(user,
            context.Options.AuthenticationType);
        ClaimsIdentity cookiesIdentity = await userManager.CreateIdentityAsync(user,
            CookieAuthenticationDefaults.AuthenticationType);
        AuthenticationProperties properties = CreateProperties(context.UserName, data["udid"]);
        AuthenticationTicket ticket = new AuthenticationTicket(oAuthIdentity, properties);
        context.Validated(ticket);
        context.Request.Context.Authentication.SignIn(cookiesIdentity);
    }
}

By simply adding

var data = await context.Request.ReadFormAsync();

within the method, you can access all posted variables in the request body and use them as you like. In my case, I placed it immediately after the null-check on the user to perform a more restrictive security check.