A carrier that only appears before vet trips quickly becomes a warning sign, and with tomorrow's appointment fixed you cannot rebuild that association from scratch. You can, however, change tonight and tomorrow morning enough to get her there safely and with far less panic than a chase and a forced entry would cause.
Tonight: turn the carrier into furniture, then a feeding spot
Set the carrier on the floor where she spends time, door wedged open so it cannot swing shut, with bedding that smells of her inside. Scatter high-value treats in and around it every hour or so and let her discover them unobserved. If she naps in it, excellent; if she only snatches treats and retreats, that is still progress. Feed her evening meal as close to the entrance as she accepts, moving the dish just inside only if she eats calmly, and never lure her in and slam the door — that single trick teaches her the carrier lies.
Keep your own behaviour part of the plan tonight: move slowly around the carrier, speak in your ordinary voice, and avoid excited relatives leaning over it to coax her, which reads to a frightened kitten as a crowd closing in. If she chooses to sleep near rather than in it, place a worn item of your clothing beside the entrance so your scent bridges the gap. The aim for this one trip is a kitten who arrives rattled but recoverable, and who eats and plays normally again once home.
Practise the moving parts, briefly and separately
Once she enters willingly, close the door for a few seconds, open it, and treat. Later, lift the carrier a few centimetres, set it down, and release her. Then a slow walk across the room and back. Each step earns food and freedom, and you stop at whichever stage makes her cry, flatten, or scramble, returning to the previous stage. Keep the whole evening's practice to short bursts between normal play, and place the carrier somewhere she can keep choosing it overnight.
Tomorrow morning: a calm, covered, single motion
Allow extra time so nobody is rushing or calling her name in a tense voice. Play with her, let her use the litter tray, and avoid a large meal immediately before travel if your vet has advised that for the appointment. When it is time, either let her walk in for treats or, if she refuses, gently lift her in backwards or lower her through a top opening without a chase around the house. Cover the carrier with a light blanket, carry it level with both hands, and secure it in the car so it cannot slide. Tell the vet team on arrival that she is frightened, so they can handle her slowly and return her to the covered carrier between checks.
After this visit, fix the association properly
Leave the carrier out permanently at home as a bed and treat spot, take occasional short car rides that end at home with a meal, and note what helped tonight — the blanket, the treats, a top opening — for next time. If her fear is extreme, with panting, drooling, or frantic injury to herself trying to escape, raise it with your veterinarian at this visit, since they can advise on stronger support for future travel. For the next trip, put a follow-up question to KinsLe's Kinny about carrier practice; he answers using the KinsLe Answers library.