A dynamic dropdown (called a reference selector) in Mendix expects to set an association between one object and another. (FYI, the Dropdown widget is only used for setting the value of an enumeration).
For example, in a typical app, you might have a Product entity and a Categories entity. Each Product may fall into a Category. You’d model this with a many-to-one association drawn from Product to Category, which then you could easily use a Reference Selector to set that reference.
So in your case, your data model has 1 SnapshotList object with many Snapshots. But you’re trying to select a single Snapshot. So, you need to add a new association from SnapshotList to Snapshot, and then use that new association in a Reference Selector widget.
I assume in the REST activity SnapshotList is the receiving object of tha REST call.
Then retrieve Snapshot over association and use that further in the MF.