After creating a Lookup field, the Apps with the Lookup field are also linked to a target App (Lookup App). In this example, the Contact App is now linked to the Customer App, but on the other hand the Customer App is not yet linked to the Contact App. In order to complete the link between the two Apps, you will need to Add Relation for the Customer App as well. 

After creating a Lookup field, follow the steps below to add Contact relation to Customer App.

1. Navigate to Customer App, then click on Relationship tab.
2. Click Add Relation.

3. Enter the relation name.
4. Select relation type and related App. If you select Bind relationship and select the Customer App, you will see Lookup Field that has been created previously available for selection.
5. Select display field (this option will show only available fields that you add in create lookup field steps).

6. Click Save.

Note: If relation has not been added, the linked records will not be available in the lookup App (target App). In this case, if you add relationship table in Field Layout, you will only see the new records that have been created after the Apps are linked but not the ones that have been created prior even if there is the Lookup field.