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