You can directly edit index.html to include the <link> using your favourite text editor.
UPDATE: More details on where to find the index.html and how to customise it can be found here – https://docs.mendix.com/howto/front-end/customize-styling-new#9-customizing-index-html-web
Hope this helps.
Where can I find index.html ?
I found it in deployment directory but it erase when I clean deployment...