I want to track internal URL that brought the URL to the next page or exit page

Question

I want to track the URL of the current page, then pass it to a URL variable to the next page.

For example 
https://www.mywebsite.com
https://www.mywebsite.com/about/?fromsource=https://www.mywebsite.com
https://www.mywebsite.com/about/careers/?fromsource=https://www.mywebsite.com/about
https://www.mywebsite.com/contact/?fromsource=https://www.mywebsite.com/about/careers

So everytime someone moves to the next page, the URL of the previous page is add to a URL variable in the next page.

Answer 1

I'm not sure if this is the right way. I would use Google Analytics API referral data. But you can create the following shortcode:

function get_current_page_link () {
    global $wp; //get global wp query
    return home_url( $wp->request ); //get the url of the current page
}
add_shortcode('get_current_page_link ', 'get_current_page_link ');

If you want to add the code via PHP code:

<a href="https://www.mywebsite.com/about/?fromsource=<?php echo do_shortcode("[get_current_page_link]"); ?>">About</a>

If you want to use it in your editor you can add url's as such:

https://www.mywebsite.com/about/?fromsource=[get_current_page_link]

The editor in the text mode, it will show up like this:

<a href="https://www.mywebsite.com/about/?fromsource=[get_current_page_link]">About</a>

Answer 2

Why not use document.referrer? So e.g. create a new variable in your web-analytics system and use the following js code to populate it:

function includeReferrer() {
  var ref = document.referrer,
      loc = document.location.href,
      newLoc;
  if (loc.includes('?') != -1) {
    newLoc = loc + "&fromsource=" + ref;
  } else {
    newLoc = loc + "?fromsource=" + ref; 
  }
}

In this case, you are not overwriting the actual URL, but instead create a new string which includes the requested parameter and can be used for any kind of tracking purpose.