Hi, go to your project directory, next go to theme/web and edit the settings.json file. Add the link to the ICOMOON to the array. Save the file and compile the project.
Mendix pages and widgets on those pages are (dynamically) loaded in the <body> of the HTML. If you want to add something to the <head> you need to do it in the HTML files
go to your project folder theme\web\index.html
and change the html file