How to implement it via iFrame?

I have a situation, when I should to send a POST request to authenticate user, if it's succeed then server set a cookie and then we can get a protected page:

1. First request by Postman:

2. Second request (after first):

I need to implement it on web page. As I understand now, it's possible only via frames. How to do it? First request should be sent automatically after opening a page, so, user should open the page and see a protected page.