I have installed a MATOMO server for tracking activity on my websites, they say it should go on every page, but I would be good with installing it on the first page after login. where if possible would this code be inserted

Make sure this code is on every page of your website. We recommend pasting it immediately before the closing </head> tag.
<!-- Matomo -->
<script>
var _paq = window._paq = window._paq || [];
/* tracker methods like "setCustomDimension" should be called before "trackPageView" */
_paq.push(['trackPageView']);
_paq.push(['enableLinkTracking']);
(function() {
var u="//millerwebtech.com/";
_paq.push(['setTrackerUrl', u+'matomo.php']);
_paq.push(['setSiteId', '1']);
var d=document, g=d.createElement('script'), s=d.getElementsByTagName('script')[0];
g.async=true; g.src=u+'matomo.js'; s.parentNode.insertBefore(g,s);
})();
</script>
<!-- End Matomo Code -->