Can you copy a creature spell on the stack and have the copy enter as a commander?

A card is your commander because it was designated as such before the game started, as defined by the Commander format rules (CR 903.3). The 'commander' status belongs to a specific physical card, not to any spell or permanent that merely shares that card's characteristics.

When you copy a spell on the stack, the copy is a new object with no relation to the original card (CR 707.2). It has no history, no zone-change tracking, and crucially, it was never designated as a commander. The copy is simply a spell that happens to have the same characteristics as your commander card.

When a copy of a creature spell resolves, it enters the battlefield as a normal creature token-like object. It will not trigger commander replacement effects (such as choosing to put it in the command zone instead of dying or being exiled), and commander damage dealt by it does not count toward the 21-damage threshold — because those rules only apply to the actual commander card (CR 903.10, 903.11).

Example: Your commander is Krenko, Mob Boss. An opponent casts Twincast copying your Krenko spell on the stack. The copy resolves and enters as a 2/2 Goblin creature — but it is NOT a commander. When it dies, it goes to the graveyard, not the command zone. Your original Krenko copy continues resolving and enters as your actual commander.

In short: commander status is a property of a designated card, not of game objects that share its printed characteristics. Copies never inherit commander status under any circumstances.