Do you mean the p1 parameter which is part of e.g. links to unsubscription page in the E-Mail?
Unfortunately those you can't generate outside of ACS as it's using a system encryption key which is not available for other use.
If you want to be able to preload data on a landing page using a custom call, you could:
- Enable "Authorize visitor identification via URL parameters" option of the landing page
- Then you can choose a field you want to match
- If it shall be encrypted, you could add a custom field where you store an encrypted string calculated with a custom encryption key
