Okay, I did some poking around, and have some preliminary findings @Arundhati Sen .
I went to the page you showed, in my browser and turned on browser developer mode. After I clicked the Trigger Snapshot -- Incremental , I noticed the browser made a call to the Share Subscriptions -- Synchronize
part of the API instead of the Create Trigger
part of the API. I thing the wording led you astray.
The Trigger is for setting up a recurring schedule to start snapshots.
API call to start a synch, what I think you really want.