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