Hi guys,
I activated Subscription Billing feature, and in Sales Order, in the Line action pane section there is a button labeled "Deferrals"

when clicked it will pop up window for Deferral's process.->

I want to modified this setting to make some default. Question is how to know which class I need to modified ? What or how is the best way to know where is the handling when form is loaded, because I think I need to modified right before the form is displayed.
I tried to create Event Handler class, in the form "SubBillDeferralTransactionDeferral", I believe this is the pop up form, event handler in their "SubBillDeferralTransactionDeferral_OnInitialized", just adding "info("hi")" and put a breakpoint, but seems it is not the right place since I cannot see of what other class or function makes the value.
I actually beginner on this development, may need some guidance of what is the best way to identify this in D365 FO development.
Thanks,