You'll need to do an API call from the endpoint to add it to a static group. You could do this using the enrollment trigger in a policy

My question is why? A Smart Group configured for computers enrolled after a fixed date would achieve the same thing. I do that all the time when I'm dishing out a change that gets included in the enrolment process, so that the change isn't re-executed on fresh enrolments that already have it.

In short a Static Group shouldn't be used for an automatic group, that said, it is possible to combine, just make sure the Smart Group includes an OR is a member of the Static Group.

Ok I see your point. Rephrasing the question: how do I get it to assign to a Smart Group during enrollment? 

It should happen automatically based on your Smart Group criteria

Smart Groups are dynamic based on your criteria you've set

I think this is correct? 

 I just did this based on an EA we already had using ModTitan's Script . we move device in and out of groups, so we created a po-up menu EA. Just choose from the drop down to move computer to group or to a new group

Looks similar to something I have so it should work

Much better, so you wanted to know about all the Macs enrolled via ADE/DEP and a specific PreStage Enrolment, and wanted to make sure fresh enrolments would automatically appear in the group (the why?).

It is easy to forget, Smart Groups are surprisingly powerful.