I want to put an icon on the tab, example a google font icon
I tried something like this (so you hopefully can see where I'm going):
"material-icons project tab-item"
You can Follow the implementation of such capabilities in this page: https://feedback.telerik.com/blazor/1419293-tab-strip-label-template. I've added your Vote for it on your behalf and I posted a modified hack that shows how you could step on some of the built-in classes and markup structure to get something similar to a template.