Your license should ideally be an encrypted value of user id, plan id, and subscription id with a secret key only you know. Don't just place all 3 next to one another as that could be too easily predictable. To do this you really need access to PHP or at least SQL where you can try to generate an encrypted key. Since you need this in CB it self my guess is your only option is to generate it using CBSubs SQL Actions (using SQL) or CB Auto Actions (using PHP or SQL). You'd also generate it at your PHP endpoint, but since you know the secret key they should be identical (can be a simple MD5, which is at least better than plan text). See the below SQL documentation for encryption functions if you just want to use CBSubs SQL Actions to do this.
dev.mysql.com/doc/refman/5.6/en/encryption-functions.html
Example as follows.
Code:
UPDATE `#__comprofiler` SET `cb_license` = MD5( '[user_id]_[plan_id]_[subscription_id]_SECRETKEY' ) WHERE `id` = '[user_id]'
Just replicate the MD5 in your script if your script needs the license as well. The above for example would store this to the field cb_license which could just be a read only text field for example.