I'm afraid you can't. Even if you buy Scriptdio Agency without the Standard Version, you will not be able to log in.
Here is why:
1. Scriptdio Standard is the main product which is either a Monthly, Yearly or One Time Subscription.
2. Scriptdio Agency is an Upgrade or Up-sell which adds the Agency features in your Scriptdio account.
3. Since Scriptdio Agency 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 Agency in your account.