Use Lookup() to retrieve the value of a field from the record referenced by a lookup field.
How to use
Lookup(lookupIDFieldApiName, targetFieldApiName)
Input
lookupIDFieldApiName: API name of the lookup field containing the ID of the referenced the record which contains the target field whose value is being retrievedtargetFieldApiName- API name of the field in the referenced record whose value to retrieve
Output
The value of the requested field.
Example
Let's assume you created a Check in button for an Account object that has a lookup field with the API name Primary_Contact in your data source referencing the Contact object, and you'd like to programmatically retrieve the first name of the primary contact (a field with the API name First_Name in the Contact object). When the Check In button is clicked for an account, you'll be able to do this with this code:
Lookup('record.Primary_Contact_ID', 'First_Name')
NOTE. Why are we using Primary_Contact_ID here?
When Mapsly imports a lookup field from a data source (for example, Primary_Contact), it creates several fields in Mapsly by adding _ID, _NAME and _TYPE to the original name of the lookup field:
Primary_Contact_ID, a field that contains the ID of the referenced record;
Primary_Contact_NAME, a field that contains the name of the referenced record (the name is retrieved from the fields mapped as Name in the referenced object in Mapsly);
[Only for polymorphic lookups] Primary_Contact_TYPE, a field containing the API name of the object referenced by the lookup field.
A polymorphic lookup is a lookup that may reference one of several objects, as opposed to a simple lookup that references the same object from all records. For example, a Salesforce object Meeting has WhoId and WhatId polymorphic lookups fields, which may reference Lead/Contact and Account/Opportunity objects, respectively.