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