Workflow Management My Inbox Extension

Dear Experts,

I have a requirement to add custom buttons for mass action of tasks located in the My Inbox of Workflow Management.

Cloud only is involved. I created a Fiori site that consists of copied My Inbox app that is reflecting assigned tasks and would want to have a button to mass approve all of them.

I googled around and everyone is suggesting to add custom code to S3 screen. But I dont know how to get to this screen to get it extended?

Any help would be appreciated.

Br,
Aleks.