Using custom authentication handler

[Vampire2008]

Hello!
I'm experimenting with IdentityServer and OpenIddict now.
I have to use my AuthenticationHandler without SignIn. It just reads a custom cookie and sets the ClaimsPrincipal. The cookie itself is created by another old app.
Don't ask why. It's just Legacy.

I got it working with OpenIddict. It just used default AuthenticationHandler for Authenticate and Challenge.

But I can't do it with IdentityServer. I reassigned DefaultScheme to my own, but it tries to SignIn for some reason.
How do I set it up correctly?

[Vampire2008]

I found solution. I just used my handler as external.

public class AccountController : Controller
{
    [HttpGet]
    public async Task<IActionResult> Login(string returnUrl)
    {
        var result = await HttpContext.AuthenticateAsync("MyScheme");

        if (!result.Succeeded)
        {
            return Challenge("MyScheme");
        }

        var identityServerUser = new IdentityServerUser(result.Principal.FindFirstValue(ClaimTypes.NameIdentifier)!)
        {
            IdentityProvider = "LegacyProvider"
        };
        await HttpContext.SignInAsync(identityServerUser);

        return Redirect(returnUrl);
    }
}