You can manage and configure the Parent-Child interlink support for “Add” Form Type. You will get the “Parent Child Link” option under the Advance Configuration accordion. 
You need to select the Add Form Type and enable the “Parent-Child Link” option. You need to select another Module as “Child Module” which should be related to the parent module.
Once the Parent-Child Link is enabled, you need to select the ‘module’ and its ‘Relationship type’ from drop-down option. You will get the Field selection as per the Relationship Type. You can drag & drop required fields to the Form builder.
Parent-Child Interlink Identification
You can set the identification field on that any Parent Module record will be identified whether that record is already existing in the Portal or not. 
You need to select the identification fields that must be String type from the Form grid.
You will find the “Create Unique Records On” dropdown list. Select the required field for unique identification and save the configuration. Once the configuration is saved, Publish the form to apply the changes.