Use an entity reference to map a D365 unique identifier into a related field in the target.
- On the Integration tab click on Additional Columns tab.
- Click on Add Additional Column button.
- The Additional Column window will open.
- Name – enter a name for the column.
- Tab off the Name field to enable the Type field.
- Type – select Entity Reference.
- Entity – select the entity that the reference is for.
- Reference – select the source column that holds the GUID for the entity reference.

- Click Save.