Damaged (?) undercarriage plastic part.

yep, that plastic did it's job.

Alternatively, if that piece of plastic turns out to be very expensive; I'd clean, duct tape it, then spray liberal amounts and several coatings of flex seal on it. A quick and dirty repair job to keep road debris from accumulating in there that may support rust formation.

Hope this helps...

 

I was going to suggest duct tape too, at least if and until you replace with either used or new.
This is the best I can do so far. I'm not sure this is the part, but it's at least getting closer than nothing.

part #'s 76974-47020-C0 or 76974-47020-C1 entering your vin might help, but there are a lot of underbody panels on my 2017 Prime and I don't see much of any of them listed while searching the 2019.

C0 part
76974-47020-C0 Genuine Toyota Cover, Rear Under Side

and the C1 part
76974-47020-C0 Genuine Toyota Cover, Rear Under Side

I don't see a diff between the two. It'd be a good idea to get under there when weather permits and pull the part. It makes it a hundred times easier to make sure you are ordering the right replacement. And if it's easy enough getting a used one you'll be sure it's the right one.

 

I"d recommend going to a dealer with the damaged part in hand. I"m not sure the link I gave you is the same part as the one that is damaged on your Prime. I'm not sure exactly what's down there, but on my 2017 there are 5 separate panels that attach to the back bumper cover alone.
The dealer parts counter should have access to the full diagram of all the under panels on your car.

If you want to DIY the panel replacement the way to get the full diagram is to get a 2 day standard subscription to TIS
https://techinfo.toyota.com/techInfoPortal
and look through the repair manual to make sure.

There's a lot more info available with a 2 day subscription than you need just to replace one silly under panel, but if the part I linked to above is the wrong under panel, I'd be sad and you'd be out at very least return shipping and restocking fee.