This is a really weird system you have devised.

You are making a custom request to ask the server to push something to the browser.

Browser: POST /preload?url=/next-page Server: PUSH /next-page

Just cut out the middleman and make a regular request.

Browser: GET /next-page

Even better use browser preloading mechanisms so that the browser knows how to best prioritize them. In fact if you do it this way the browser can even start downloading subresources and prerendering the page.