This has been covered quite a few times in this forum. Long story short, use the web API to look up the customer either using an email, or using some FastSpring provided unique identifier and store that in a custom license field.
Longer explanations / examples:
https://wyday.com/forum/t/2413/subscriptions/#post-12239
https://wyday.com/forum/t/4270/fastspring-and-saas/#post-20196
https://wyday.com/forum/t/4419/cannot-rebill-license-using-fastspring-script/#post-20767