Can I Use Bet.com/Activate on Firestick?

Yes, you are able to use Bet.com/Activate on Firestick . Here are the steps that you have to follow:
On your Amazon fire TV and go to the app store in your device
Then search BET app and install it in your device
Complete the sign in process and get activation code
Go to bet.com/activate in another device
And enter that code and click activate
That's it.