I want to tie an ID to an account and have it populate a field in all leads

Hello!

I want to give some of my accounts that are for my affiliates an ID so when a lead is created they have a hidden field with an Affiliate ID, then using the affiliate ID i would autofill a field with the Account of who has that affiliate ID. Ive tried lookups but it wont pull up a custom field.

HELPP!

Thanks