I'm afraid you can't. Even if you buy ExplaindioPlayer Agency without the Standard Version, you will not be able to login.
Here is why:
1. ExplaindioPlayer Standard is the main product which comes as a Monthly or Yearly Subscription.
2. ExplaindioPlayer Agency is an Upgrade or Up-sell which adds the Agency features for your ExplaindioPlayer Software.
3. Since ExplaindioPlayer Agency is not independent, you will need to subscribe for the main product a Monthly or Yearly Subscription of first ExplaindioPlayer. Else you can't use the software by only having ExplaindioPlayer Agency in your account.