[This is with the SOAP API]
The pattern we use is to add a large number of leads to Marketo with SyncLeads using a key of our client's choosing, then sometime later add specific leads to a list. Because ListOperation can only take the "IDNUM", we fetch the Marketo Id using GetMultipleLeads and the key we originally used with SyncLeads.
Previously, we've only supported email, marketo id and salesforce, but now we're looking to add the "Custom" value which some of our clients use already. This was simple enough to do with SyncLeads (ForeignSysType.CUSTOM the way we've already used ForeignSysType.SFDC), but how do I use the value I used in SyncLeads in GetMultipleLeads? None of the LeadKeyRefs seem to match up.