Currently, a trigger which gets displayed on the Atlas UI (under Data Services > Triggers) cannot be created programmatically, they may only be created by clicking through the Atlas UI.

The triggers created from the Atlas UI automatically creates a new realm app named "TriggersRealmApp" even when a realm app with the same name already exists. There is no way to have any of the triggers created under any other realm app displayed in the Atlas UI. The only way is to create them under the auto generated "TriggersRealmApp" which gets created after the first trigger from the Data Services > Triggers Atlas page.