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