Ah, you would see the code in the "View frame source" if the HTML Sidebar was minimized or open. What the HTML Sidebar does is wrap the page content into its own iframe, which is why the "View frame source" option is available when the HTML Sidebar is open or minimized, but disappears when it is closed. You can close the HTML Sidebar by clicking on the "CLOSE" button on the bottom left corner of the dialog.
This will allow you to see the tracking code from the "View page source", rather than "View frame source". Also, as seen from your screenshot, it looks like your tracking code is properly inside the < head >, as it is above the closing < /head > tag (right above the < body > tag).
Further, I just tested adding a FullStory plugin to my Axure Share prototype, and it seemed to work when the HTML Sidebar was completely closed. When the HTML Sidebar was open or minimized, it didn't work. I looked into this, and it looks like content in iframes for FullStory will need an extra line of code for it to work properly. If you need the prototype to work with the HTML sidebar open, minimized, or closed, you will want to add the following line of code to your plugin:
window['_fs_run_in_iframe'] = true;
And here's a screenshot to demonstrate where you can add this line in the tracking code:
You can read about this in FullStory's help page here:
Hopefully this helps!