@Galaxiom
Right, I looked at your demo file and see what you're doing there. I like that approach, may come in handy some time. However, it still requires at least some level of user interaction (i.e. the user needs to select certain values in the main form before other values in the subform can be "pre-filled").
That is what I'm not clear on in terms of what the OP wants. They seem to be saying (and I certainly could be misunderstanding) that they want to somehow pre-fill one or multiple records without any type of user interaction ahead of time. They haven't stated whether this is based on previous selections in another form, or some other scenario.