Pre-form fills using API

[Jessica_Ruffalo]

Has anyone used preform fills using API? How did you do that? 

We are looking to create this for our subscription center in our website and want to ensure that when someones email address is populated, their subscription settings are visible.  We know Marketo has the cookies but we are looking for a permanent option incase the user clears their cache or is using a different browser, device, etc.