GameMaker Studio 2 Mod Loader
(Based on Cgytrus's GMML, made because GMML is broken on newer GameMaker versions due to a change in how the exe is compiled.)
While documentation is a work in progress, it should have enough info here to get you started.
https://omegametor.github.io/GS2ML/
Having trouble? Join the GS2ML discord and ask for help!
https://discord.gg/ejDDx3ady3
https://github.com/cgytrus/gmml
which is based on
https://github.com/krzys-h/UndertaleModTool
-Finish documentation
-(MAYBE) find a way to do C# interop without having to hook into the exe and break every time the runtime updates like GMML did. Not sure if this is even possible, lmao.