How to trigger a business process on click of " + " button in a detail

Dear Sri,

When you adding a record on detail, you basically adding a record to an object. You can create a business process, which launches by signal of adding a record to the object, on which detail is based.

Other approach requires development. You can override basic onClick method for + sign - "addRecord" on the schema of your detail. Firstly, call parent realization, after that run process launch. 

More on process call from client side can be found here: 

https://academy.bpmonline.com/documents/technic-sdk/7-13/process-launch-client-module

Regards,

Anastasia