Journey Builder Sales and Service Activity Find and Update Fields

By nature, any ID field will have an IdLookup property set to TRUE. For example, Account ID, Lead  ID, Contact ID will all be able to be used as a lookup field when updating those respective objects. For Standard Objects, check out the developer documentation that lists additional standard fields with this property. 

If a field is not available for lookup, you will need to create a new field or edit an existing field in your Sales/Service org. To do so in Salesforce Classic, have an Admin at your organization go to Setup | Customize | Click the Object in question | Fields | Click Edit on the existing field. Check the "External ID" box and save. The field should now show as an option when configuring an activity to Update a record in Sales/Service. 

Create a Custom Field Trailhead

Differences between the 'External ID' field and the 'Unique ID' field setting