COMPLETE EDIT (since nobody understands what I'm saying): I have downloaded the game "Undertale" on the Linux Steam client using this method:
https://github.com/dotfloat/steam-appmanifestthis method.
However, Steam recognizes that the game does not run natively on Linux and will not allow me to start "Undertale" via a custom launch with Wine.
Is there a workaround that, for example, tricks the Linux Steam client into thinking that "Undertale" does run natively on Linux so that I can open "Undertale" through the Linux Steam client?
ALSO: I am currently using the Windows Steam client through Wine; I am writing this because I DO NOT want to use the Windows Steam client.