Yeah the ArcGIS feature for iTAK let's you view any layer of your choice, it's pretty powerful but does not offer a data capture/upload capability like Field Maps does, it's read only. The feature worked with NIFC AGOL last year, but I just checked and it's broken for me too. I suspect that has to do with the 2 factor auth requirement that they added this year, but I'll ask the iTAK developers to look into it. In the meantime if there are other ArcGIS layers that you'd like to see in TAK just let me know and I'll see if we can integrate them similar to our existing fire channels.