How do I appeal an out of warranty diagnosis?

Long story short:

On January 4th, I contacted tech support because the right shift key stopped working on my Inspiron 7577 gaming laptop. It was diagnosed via remote connection and determined that I needed to ship it back to Dell for repairs.  This laptop is under a year old, and is still under warranty.

Fast forward to January 17th - My Dell repair status was updated:  "1/17/2019 1:39:32 PM: Billable Hold: The repair required for your system is not covered by your current warranty and will require additional payment."

I called the out of warranty service department.  They told me the repairs aren't covered because of liquid spillage on my keyboard, and it would cost me an additional $149.  This diagnosis is incorrect and false. My laptop is in like new condition aside from the right shift key.  I have taken great care to ensure incidents like this do not happen.  Liquid has never been spilled or dripped onto this laptop.

How do I escalate this to appeal the diagnosis?  I have little faith that I will get satisfaction from the rank-and-file.

Thanks!