That plan already comes with unlimited 3G Hotspot.
Now if you do wish to add more you have the T-Mobile One Plus feature which comes with 10GB of 4G LTE Hotspot.
Or the other option if you need the unlimited 4G LTE add the T-Mobile One Plus International feature which brings UNLIMITED 4G LTE Hotspot.
You can add these right from your T-Mobile app or by calling customer care .
Manage your T-Mobile account