Create a Field Mapping
Merge gives you flexibility in how origin fields and targets are constructed, depending on your use case and intended user experience.
There are two ways a Field Mapping can be created:
For All Linked Accounts Across an Integration
Integration-wide mappings allow you to map standard app fields that are not Common Model fields to all Linked Accounts for that app.
For a Specific Linked Account
Linked Account-level mappings allow you to map a custom field that is specific to a Linked Account.