Read through the following section to understand the Scribe map to update a CRM lookup field. It is not always a simple drag-n-drop mapping. Lookup fields in the CRM store a reference ID (GUID) to the row in a different entity, so it's critical that the ID is ultimately sent to CRM and not some text value.

Consider the example of Source Campaign, which is a standard lookup field on the Lead entity in Dynamics CRM. The following use case here is marketing wants to UPDATE this field over in Dynamics CRM. Acoustic Campaign is the source of the mapping.

Campaign Name: Amplify 2016

Campaign ID/GUID: 208d4ae8-5f3d-e211-8603-00155d2a0205

The following are three mapping techniques that would also apply to most any lookup fields.

1. If you have the campaignid (GUID) of the campaign record to which you want to map to CRM for the Lead, you can put the campaignid (GUID) in a Acoustic Campaign custom field and map it directly to the CRM lookup field. No transformations needed. You are passing over exactly what the field stores in CRM. Effective, it means you marketing team is working with GUIDS, which are not easily identifiable. Also, might not be practical if the users or the contacts are selecting it from a Selection (choose one) field in Acoustic Campaign.
2. If you are collecting the text values (Campaign Name) in Acoustic Campaign, then you could use a Scribe formula. Here's an example. IF(ISNULLORWHITESPACE(MSCRM_Lead.CRMCampaign), null, LOOKUPTARGETVALUE('campaign','name', MSCRM_Lead.CRMCampaign, 'campaignid' )). You cannot use campaign names that do not exist already in CRM.
3. Another option is to use the Scribe Online Lookup block to perform a lookup to CRM to retrieve the campaignid, which is mapped to the CRM campaignid field. Similar to #2, you might be collecting the Campaign Name in Acoustic Campaign, so the Scribe Lookup block can look up the Campaign Name that is provided by IMC to the CRM Campaign entity and the return value can be the campaignid. Which you would then map to the lookup field in CRM. As with #2, be familiar with your data and be sure that marketing is using campaign names that exist in CRM.