I have had some experience of this when I was in my early twenties although I wasn't agoraphobic, but I did have some panic attacks. I read up a lot about it and had a couple of CBT sessions. And I helped my teen DD when she got nervous and panicky before exams.
Totally agree with Rtmhwales with the only addition from my experience being that you go out for a "purpose" , even if it's fabricated, otherwise it will be on your dd's mind that she is going out for the sake of going out to cure her problem, and all thoughts become focused on her panic, whereas if you go out for a purpose (not easy atm I realise) then she will be hopefully distracted from it a bit (it's very very important that she learns to diminish her panic too - through calm breathing - and stay with it until it diminishes and not retreat from it) and ultimately she will have the added bonus that it is a pleasurable experience, if she does something she enjoys.
For example, if she likes baking, could you support her to go to a corner shop to buy ingredients? And then you bake cakes together = pleasurable reward.
Could you accompany her on a 10 minute run, or a bike ride or a skate boarding session in the park? Maybe challenge one another to run, cycle a short distance supposedly for fitness purposes?
You need to start very small and build up gradually.
Maybe go out for a drive first to pick up some take away food?
Then go with her to the shop to buy a magazine.
A couple of days later back to the takeaway and she gets out of the car to pay or to buy drinks or similar?
Next day, back to the corner shop to buy two more magazines and add a five minute walk to the chemist?
Then go to a bus or train station and get her to go one stop and you pick her up at the other end.
The next day she does two stops etc etc.
Make the outings pleasurable, calm and upbeat. Put music of her choice on. Let some of them end with a treat - go to buy a new t-shirt?
Try and talk to her very gently about the detailed specifics of her fear. What is it exactly that is making her anxious? Is it that she might have a panic attacks, or she might cry or throw up or embarrass herself in public? Is it catching Covid or giving it to you? If so, then reassure her that loads of people go through the same thing, that she is totally normal, everyone is focused on their own business and won't be looking at her, that she can recover if she practices every day, and keep emphasising calmly that what she is experiencing is nothing to panic about in itself (as teens can become fearful of the fear itself) and these are worrying times but she will be fine. Keep very very calm yourself and don't show any distress at her panic. Be very reassuring.
Something that might help her to get out of the door in the first place is getting her to practice breathing exercises and learning how to calm herself down before she goes outside , maybe start in your hallway, or just in your garden, or outside your house. There are lots of You Tube videos about this. Have a look at Therapy in a Nutshell - there are free videos and also a reasonably priced course you can follow together. It's good because it's sympathetic yet backed up with science.
A friend of mine learned to diminish their own panic on aeroplanes by learning Tibetan breathing from You Tube (sounds strange but it worked well).
If your daughter likes animals, then I would highly recommend trying to book her a session at a hippotherapy or equine therapy centre, where she can learn to bring herself down from a high state of panic to a state of calm, by being around horses and learning about their reactions to anxiety (they react to anxiety in humans too so it's a great way to learn) and indeed the entire art of horse training is about going from a state of flight ( or very occasionally fight ) to a state of calm, so there are many parallels.
Your gp may be able to recommend some calming pills, natural remedies that you can buy over the counter at the chemist, there are quite a few on the market, even the placebo effect can help. When she is older, she may be able to take beta blockers to manage the symptoms but she's probably a bit young for those yet
One last thing - once your daughter gets more confident about going out and the weather gets better - think about doing a family on-line walking challenge such as The Conqueror Online Fitness Challenge www.theconqueror.events/ . I know that's way off what she can manage now but it's good to have an objective you can build up to, and mentioning it as something you can maybe do later on, shows you have complete faith in her that she can do it. That's for much later though; best to start very very small for now. I only mention it because what helped me immensely was my DH treating me totally normally as if I didn't have a problem with anxiety at all when we spoke about the future, even though he was sympathetic to when I had a panic attacks in the present if that makes sense. But we never started out anywhere with him assuming I was going to have one; we started each day afresh and that really really helped me.
Btw, do you have a dog, because they can sometimes help and reassure anxious DC? You could perhaps approach the Cinnamon Trust in your area and borrow one if not.
Sorry for long post! Good luck, these are really hard times for teens and I hope your lovely girl recovers her equilibrium soon. 
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