Call custom function within tracking code

Our old PIWIK instance was a self install on a server where we edited the piwik.js file to append some custom javascript at the end of the file to increase the data we got from the tracking code

We now have a MATOMO cloud instance where we can’t edit the piwik.js file

How do I call the function from the tracking code that I have saved in a file called custom_piwik.js which is stored in a folder on my server ??