Ex:
Entity1 is related to Entity2 in 1:N fashion. Entity1 will contain subgrid and Entity2 will contain Entity1 lookup
To enable create on click of "+" sign
- Go to related lookup field in the related entity - Entity1 lookup field in Entity2
- Customize the field and make it mandatory
- If the field is mandatory, then automatically the "+" sign will point to create of new record
To enable add existing on click of "+" sign
- Go to related lookup field in the related entity - Entity1 lookup field in Entity2
- Customize the field and make it non-mandatory
- If the field is mandatory, then automatically the "+" sign will point to existing record
Note: This behavior is not applicable to some of the OOB relationships.