I'm afraid you can't. Even if you buy WaveNetVocalizer Agency without the Standard Version and even if you buy without the Standard Version you will not be able to login.
Here is why:
1. WaveNetVocalizer Standard is the main product which is either an Yearly Subscription or Monthly Subscription.
2. WaveNetVocalizer Agency is an Upgrade or Up-sell which adds the Agency features in your WaveNetVocalizer account.
3. Since WaveNetVocalizer Agency is not independent, you will need to subscribe for the main product WaveNetVocalizer yearly or monthly subscription first. Else you can't login with only WaveNetVocalizer Agency in your account.