API - Use Custom Field to Access Leads

[Anonymous]

Hello, we are just trying to see if there is a way we can use a custom field to access leads using marketo's SOAP API, rather than lead ID.

Issue
The API only appears to use the Marketo Id to identify leads.
We do not store the Marketo Id in the place Marketo is passing the leads to. 
 
Ideal solution
We want to be able to use a custom field to identify leads.
However, it appears the API will not allow us to use this custom field.
 
I would appreciate any help! Thank you. 

[Anonymous]

Currently this is not possible. List IDs are proably best to way to update or retrieve leads in mass via the APIs.

In terms of a custom field, you can use foreign key, and write to leads based on that. I am not sure if that would helpful in this scenario.

http://developers.marketo.com/blog/storing-a-foreign-key-in-marketo/
http://developers.marketo.com/documentation/soap/syncmultipleleads/