I' I'm afraid you can't. Here is why:
1. Speedlir Agency is an upgrade of which adds the Agency Module in your Speedlir Account.
2. Since Speedlir Agency is only up-sells or upgrade you must have Speedlir FE (Standard) in your account first in order to buy the Speedlir Agency.
3. Even if you buy Speedlir Agency without buying the main Speedlir subscription you wouldn't be able to use Speedlir app because Speedlir Agency is dependent on the standard product. So, with only Speedlir Agency in your account, you wouldn't be able to login.
Note: In case you bought Speedlir Agency without buying Speedlir consider buying the standard/main product from our sales page here: http://speedlir.com/fe/