I don't think there's any way to configure the value for the forms cookies through any kind of forms configuration though there are other options available. One way would be to set a cookie policy in your startup.cs like this (though bear in mind it will be applied to all cookies):

app.UseCookiePolicy(
    new CookiePolicyOptions
    {
        MinimumSameSitePolicy = SameSiteMode.Lax,
        Secure = CookieSecurePolicy.SameAsRequest,
        HttpOnly = Microsoft.AspNetCore.CookiePolicy.HttpOnlyPolicy.Always
    });