First off, your powertrain warranty was potentially voided due to this, so bear this in mind with anything you do. That said, it sounds like the engine may have been hydrolocked in this case and may be dead. The first thing you want to do is pull the spark plugs and fuel pump relay (leave the starter relay in) and do 2-3 fake starts to get the water out of the engine, and give replacing them some serious thought if they look like they're compromised. Doing this is pretty easy on the i4 Hyundai engines like the Forte's once you remove the beauty cover. Change the oil as well in the engine; use some cheap SAE certified oil like budget oil from Walmart just in case the engine is toast.
Don't worry about using premium (iridium coated) sparkplugs on a i4 FWD car. This isn't a European luxury car or Lexus which needs premium gas and iridium sparkplugs. This is a Hyundai i4, likely 2-3L. It will do nothing but add cost on a Forte.
Refer to this answer for what to do as a starting point to best preserve the engine: went through a foot of water and it stopped working - Kia Forte - iFixit
In the event the engine is hyrolocked, you gotta replace it unless you can rebuild it for less than a replacement and do it reliably; most places cannot rebuild modern engines correctly anymore. You can probably go a 3rd party to replace it for less (likely not by much due to the raw cost of replacement engines) or if you know how, you can potentially swap the engine at the DIY level, albeit considering it's an advanced repair. It's not for the faint of heart, DIY engine swaps should be done by someone who can work on their own vehicles and confidently do it.
The other caution I have is the ATF will need to be from Hyundai, and you cannot use generic fluid. Preserve the old fluid (be careful not to lose any) in clean bottles just in case there were particles keeping it running smoothly, but I wouldn't be too fussy if you lose too much with your low milege; this isn't a 100k+ mile car where you basically lose the transmission if you don't keep the original fluid. You'll also want to do this in case they use the BMW model of selling it "dealer only" by the transmission volume so you can drive the car.
Since the powertrain warranty may have been voided (especially without any clear answer) I would take it to another dealer after trying those steps. If they refuse coverage twice, it's painfully obvious or voided. PLAY DUMB.
@lollylgb what have you checked? Does it crank over? Do you have spark at the plugs? Give us more details of what your Kia currently does.