You don't have to be a BT customer to use their non contract monthly BT TV Sports service.
If you are wanting to watch the Fury v Wilder boxing event, then this is on BT Sports Box Office Pay per View and not covered under the BT Sports service linked above.
Have a read of the link that i posted, where BT Sports is a totally separate service and you subscribe and pay BT Sports directly.
There is a new customer register tab on the link to create a BT Sports non contract monthly account.
Like @schnapps is saying only Republic of Ireland customers can add BT sports to NOW.
Here in the UK you have to take BT out directly with them, which will operate on a separate app.