Hi Partner,
You could add custom JS code in Home webpage to do this.
Please refer to the following steps.
1.Go to Dynamics 365 Portal Management and find your Home web page for your website.

2.Open the record and in "Localized Content" session, open the "Home" record again.

3.In "Advanced" tab, use the following JS code.

$(function () {
var url = document.location.toString();
var arrUrl = url.split("//");
var start = arrUrl[1].indexOf("/");
var relUrl = arrUrl[1].substring(start 1,start 7);
if(relUrl==null||relUrl==""||relUrl==undefined||relUrl!="SignIn"){
window.location.href="SignIn";
}
})
Save and wait the settings sync to Portal.
Then when you go to the portal URL directly, the page will be redirected to SignIn page as you wish.
Regards,
Leo