I'm afraid you can't. Even if you buy Speechdio PRO 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 PRO is an Upgrade or Up-sell which adds the PRO features in your Speechdio account.
3. Since Speechdio PRO 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 PRO in your account.