var claims = new List<Claim>
{
    new Claim(ClaimTypes.Name, "SomeUserName")
    // ... other stuffs
};

var claimsIdentity = new ClaimsIdentity(
    claims, CookieAuthenticationDefaults.AuthenticationScheme);

var authProperties = new AuthenticationProperties();

var claimsPrincipal = new ClaimsPrincipal(claimsIdentity);

await HttpContext.SignInAsync(
    CookieAuthenticationDefaults.AuthenticationScheme,
    claimsPrincipal,
    authProperties);

services.AddAuthentication(CookieAuthenticationDefaults.AuthenticationScheme)
    .AddCookie(options =>
    {
        options.Cookie.SameSite = SameSiteMode.None;
        options.EventsType = typeof(CustomCookieAuthenticationEvents);
    });

// ...
app.UseAuthentication();