I am not sure what you mean by "manually", but for multi-assignment mode it is mandatory to use AssignmentStore approach and don't use resourceId field. So, yes, you must have records in AssignmentStore to make it work.
Ok, thanks for your quick response.
Only in order to clarify and understanding, If I have to manage a list of AssignmentModel for multi-assignment mode, what is this function for? https://www.bryntum.com/products/scheduler/docs/api/Scheduler/model/mixin/EventModelMixin#function-assign
If I'm right, if I supply this function one list of resourceID's which I want to assign the event, and my event model hasn't got resourceId property filled, this should add internally in bryntum a list of assignment models relating the event model and the resourcesIds I supply, without having to manage an Assignment Model list on my own and without having to add it to the AssignmentStore manually.
Please, let me know your thought.